705. Design HashSet - Explanation

Description

Design a HashSet without using any built-in hash table libraries.

Implement MyHashSet class:

void add(key) Inserts the value key into the HashSet.
bool contains(key) Returns whether the value key exists in the HashSet or not.
void remove(key) Removes the value key in the HashSet. If key does not exist in the HashSet, do nothing.

Example 1:

Input: ["MyHashSet", "add", "add", "contains", "contains", "add", "contains", "remove", "contains"]
[[], [1], [2], [1], [3], [2], [2], [2], [2]]

Output: [null, null, null, true, false, null, true, null, false]

Explanation:
MyHashSet myHashSet = new MyHashSet();
myHashSet.add(1); // set = [1]
myHashSet.add(2); // set = [1, 2]
myHashSet.contains(1); // return True
myHashSet.contains(3); // return False, (not found)
myHashSet.add(2); // set = [1, 2]
myHashSet.contains(2); // return True
myHashSet.remove(2); // set = [1]
myHashSet.contains(2); // return False, (already removed)

Constraints:

0 <= key <= 1,000,000
At most 10,000 calls will be made to add, remove, and contains.

Topics

Company Tags

Please upgrade to NeetCode Pro to view company tags.

Prerequisites

Before attempting this problem, you should be comfortable with:

Hash Functions - Understanding how keys map to bucket indices via modulo operations
Linked Lists - Implementing node-based structures for collision handling (separate chaining)
Binary Search Trees - Understanding BST operations for an optimized collision resolution approach
Bit Manipulation - Using bitwise operations (OR, AND, XOR) for space-efficient storage

1. Brute Force

Intuition

The simplest implementation uses a dynamic array to store all keys. For each operation, we search through the array linearly. This works correctly but is inefficient since every operation requires scanning potentially all stored elements.

Algorithm

Initialize an empty array data.
For add(key): If the key is not already in the array, append it.
For remove(key): If the key exists in the array, remove it.
For contains(key): Return true if the key exists in the array.

class MyHashSet:

    def __init__(self):
        self.data = []

    def add(self, key: int) -> None:
        if key not in self.data:
            self.data.append(key)

    def remove(self, key: int) -> None:
        if key in self.data:
            self.data.remove(key)

    def contains(self, key: int) -> bool:
        return key in self.data

public class MyHashSet {
    private List<Integer> data;

    public MyHashSet() {
        data = new ArrayList<>();
    }

    public void add(int key) {
        if (!data.contains(key)) {
            data.add(key);
        }
    }

    public void remove(int key) {
        data.remove(Integer.valueOf(key));
    }

    public boolean contains(int key) {
        return data.contains(key);
    }
}

class MyHashSet {
private:
    vector<int> data;
public:
    MyHashSet() {}

    void add(int key) {
        if (find(data.begin(), data.end(), key) == data.end()) {
            data.push_back(key);
        }
    }

    void remove(int key) {
        auto it = find(data.begin(), data.end(), key);
        if (it != data.end()) {
            data.erase(it);
        }
    }

    bool contains(int key) {
        return find(data.begin(), data.end(), key) != data.end();
    }
};

class MyHashSet {
    constructor() {
        this.data = [];
    }

    /**
     * @param {number} key
     * @return {void}
     */
    add(key) {
        if (!this.data.includes(key)) {
            this.data.push(key);
        }
    }

    /**
     * @param {number} key
     * @return {void}
     */
    remove(key) {
        const index = this.data.indexOf(key);
        if (index !== -1) {
            this.data.splice(index, 1);
        }
    }

    /**
     * @param {number} key
     * @return {boolean}
     */
    contains(key) {
        return this.data.includes(key);
    }
}

public class MyHashSet {
    private List<int> data;

    public MyHashSet() {
        data = new List<int>();
    }

    public void Add(int key) {
        if (!data.Contains(key)) {
            data.Add(key);
        }
    }

    public void Remove(int key) {
        if (data.Contains(key)) {
            data.Remove(key);
        }
    }

    public bool Contains(int key) {
        return data.Contains(key);
    }
}

type MyHashSet struct {
    data []int
}

func Constructor() MyHashSet {
    return MyHashSet{data: []int{}}
}

func (this *MyHashSet) Add(key int) {
    if !this.Contains(key) {
        this.data = append(this.data, key)
    }
}

func (this *MyHashSet) Remove(key int) {
    for i, v := range this.data {
        if v == key {
            this.data = append(this.data[:i], this.data[i+1:]...)
            return
        }
    }
}

func (this *MyHashSet) Contains(key int) bool {
    for _, v := range this.data {
        if v == key {
            return true
        }
    }
    return false
}

class MyHashSet() {
    private val data = mutableListOf<Int>()

    fun add(key: Int) {
        if (!contains(key)) {
            data.add(key)
        }
    }

    fun remove(key: Int) {
        data.remove(key)
    }

    fun contains(key: Int): Boolean {
        return data.contains(key)
    }
}

class MyHashSet {
    private var data: [Int]

    init() {
        data = []
    }

    func add(_ key: Int) {
        if !contains(key) {
            data.append(key)
        }
    }

    func remove(_ key: Int) {
        if let index = data.firstIndex(of: key) {
            data.remove(at: index)
        }
    }

    func contains(_ key: Int) -> Bool {
        return data.contains(key)
    }
}

Time & Space Complexity

Time complexity: $O(n)$ for each function call.
Space complexity: $O(n)$

2. Boolean Array

Intuition

Since keys are constrained to [0, 1000000], we can use direct addressing with a boolean array. The index represents the key, and the boolean value indicates presence. This provides O(1) operations but uses fixed memory regardless of how many keys are stored.

Algorithm

Initialize a boolean array of size 1000001, all set to false.
For add(key): Set data[key] = true.
For remove(key): Set data[key] = false.
For contains(key): Return data[key].

class MyHashSet:

    def __init__(self):
        self.data = [False] * 1000001

    def add(self, key: int) -> None:
        self.data[key] = True

    def remove(self, key: int) -> None:
        self.data[key] = False

    def contains(self, key: int) -> bool:
        return self.data[key]

public class MyHashSet {
    private boolean[] data;

    public MyHashSet() {
        data = new boolean[1000001];
    }

    public void add(int key) {
        data[key] = true;
    }

    public void remove(int key) {
        data[key] = false;
    }

    public boolean contains(int key) {
        return data[key];
    }
}

class MyHashSet {
private:
    vector<bool> data;
public:
    MyHashSet() : data(1000001, false) {}

    void add(int key) {
        data[key] = true;
    }

    void remove(int key) {
        data[key] = false;
    }

    bool contains(int key) {
        return data[key];
    }
};

class MyHashSet {
    constructor() {
        this.data = new Array(1000001).fill(false);
    }

    /**
     * @param {number} key
     * @return {void}
     */
    add(key) {
        this.data[key] = true;
    }

    /**
     * @param {number} key
     * @return {void}
     */
    remove(key) {
        this.data[key] = false;
    }

    /**
     * @param {number} key
     * @return {boolean}
     */
    contains(key) {
        return this.data[key];
    }
}

public class MyHashSet {
    private bool[] data;

    public MyHashSet() {
        data = new bool[1000001];
    }

    public void Add(int key) {
        data[key] = true;
    }

    public void Remove(int key) {
        data[key] = false;
    }

    public bool Contains(int key) {
        return data[key];
    }
}

type MyHashSet struct {
    data []bool
}

func Constructor() MyHashSet {
    return MyHashSet{data: make([]bool, 1000001)}
}

func (this *MyHashSet) Add(key int) {
    this.data[key] = true
}

func (this *MyHashSet) Remove(key int) {
    this.data[key] = false
}

func (this *MyHashSet) Contains(key int) bool {
    return this.data[key]
}

class MyHashSet() {
    private val data = BooleanArray(1000001)

    fun add(key: Int) {
        data[key] = true
    }

    fun remove(key: Int) {
        data[key] = false
    }

    fun contains(key: Int): Boolean {
        return data[key]
    }
}

class MyHashSet {
    private var data: [Bool]

    init() {
        data = [Bool](repeating: false, count: 1000001)
    }

    func add(_ key: Int) {
        data[key] = true
    }

    func remove(_ key: Int) {
        data[key] = false
    }

    func contains(_ key: Int) -> Bool {
        return data[key]
    }
}

Time & Space Complexity

Time complexity: $O(1)$ for each function call.
Space complexity: $O(1000000)$ since the key is in the range $[0, 1000000]$ .

3. Linked List

Intuition

To reduce memory while handling collisions, we use separate chaining. An array of buckets stores linked lists, and keys are assigned to buckets using a hash function. Each operation traverses only the linked list in the relevant bucket, making average-case operations faster than the brute force approach.

Algorithm

Initialize an array of 10000 buckets, each with a dummy head node.
Define hash(key) as key % 10000.
For add(key): Traverse the list at hash(key). If the key already exists, return. Otherwise, append a new node with the key.
For remove(key): Traverse the list at hash(key). If a node with the matching key is found, remove it by updating the previous node's next pointer.
For contains(key): Traverse the list at hash(key). Return true if the key is found, false otherwise.

class ListNode:
    def __init__(self, key: int):
        self.key = key
        self.next = None

class MyHashSet:

    def __init__(self):
        self.set = [ListNode(0) for _ in range(10**4)]

    def add(self, key: int) -> None:
        cur = self.set[key % len(self.set)]
        while cur.next:
            if cur.next.key == key:
                return
            cur = cur.next
        cur.next = ListNode(key)

    def remove(self, key: int) -> None:
        cur = self.set[key % len(self.set)]
        while cur.next:
            if cur.next.key == key:
                cur.next = cur.next.next
                return
            cur = cur.next

    def contains(self, key: int) -> bool:
        cur = self.set[key % len(self.set)]
        while cur.next:
            if cur.next.key == key:
                return True
            cur = cur.next
        return False

public class MyHashSet {

    private static class ListNode {
        int key;
        ListNode next;

        ListNode(int key) {
            this.key = key;
        }
    }

    private final ListNode[] set;

    public MyHashSet() {
        set = new ListNode[10000];
        for (int i = 0; i < set.length; i++) {
            set[i] = new ListNode(0);
        }
    }

    public void add(int key) {
        ListNode cur = set[key % set.length];
        while (cur.next != null) {
            if (cur.next.key == key) {
                return;
            }
            cur = cur.next;
        }
        cur.next = new ListNode(key);
    }

    public void remove(int key) {
        ListNode cur = set[key % set.length];
        while (cur.next != null) {
            if (cur.next.key == key) {
                cur.next = cur.next.next;
                return;
            }
            cur = cur.next;
        }
    }

    public boolean contains(int key) {
        ListNode cur = set[key % set.length];
        while (cur.next != null) {
            if (cur.next.key == key) {
                return true;
            }
            cur = cur.next;
        }
        return false;
    }
}

class MyHashSet {
private:
    struct ListNode {
        int key;
        ListNode* next;
        ListNode(int k) : key(k), next(nullptr) {}
    };

    vector<ListNode*> set;

    int hash(int key) {
        return key % set.size();
    }

public:
    MyHashSet() {
        set.resize(10000);
        for (auto& bucket : set) {
            bucket = new ListNode(0);
        }
    }

    void add(int key) {
        ListNode* cur = set[hash(key)];
        while (cur->next) {
            if (cur->next->key == key) {
                return;
            }
            cur = cur->next;
        }
        cur->next = new ListNode(key);
    }

    void remove(int key) {
        ListNode* cur = set[hash(key)];
        while (cur->next) {
            if (cur->next->key == key) {
                ListNode* temp = cur->next;
                cur->next = temp->next;
                delete temp;
                return;
            }
            cur = cur->next;
        }
    }

    bool contains(int key) {
        ListNode* cur = set[hash(key)];
        while (cur->next) {
            if (cur->next->key == key) {
                return true;
            }
            cur = cur->next;
        }
        return false;
    }
};

class ListNode {
    /**
     * @param {number} key
     */
    constructor(key) {
        this.key = key;
        this.next = null;
    }
}

class MyHashSet {
    constructor() {
        this.set = Array.from({ length: 10000 }, () => new ListNode(0));
    }

    /**
     * @param {number} key
     * @return {number}
     */
    hash(key) {
        return key % this.set.length;
    }

    /**
     * @param {number} key
     * @return {void}
     */
    add(key) {
        let cur = this.set[this.hash(key)];
        while (cur.next) {
            if (cur.next.key === key) {
                return;
            }
            cur = cur.next;
        }
        cur.next = new ListNode(key);
    }

    /**
     * @param {number} key
     * @return {void}
     */
    remove(key) {
        let cur = this.set[this.hash(key)];
        while (cur.next) {
            if (cur.next.key === key) {
                cur.next = cur.next.next;
                return;
            }
            cur = cur.next;
        }
    }

    /**
     * @param {number} key
     * @return {boolean}
     */
    contains(key) {
        let cur = this.set[this.hash(key)];
        while (cur.next) {
            if (cur.next.key === key) {
                return true;
            }
            cur = cur.next;
        }
        return false;
    }
}

public class ListNode {
    public int Key;
    public ListNode Next;

    public ListNode(int key) {
        Key = key;
        Next = null;
    }
}

public class MyHashSet {
    private ListNode[] set;

    public MyHashSet() {
        set = new ListNode[10000];
        for (int i = 0; i < set.Length; i++) {
            set[i] = new ListNode(0); // Dummy head
        }
    }

    public void Add(int key) {
        ListNode cur = set[key % set.Length];
        while (cur.Next != null) {
            if (cur.Next.Key == key) return;
            cur = cur.Next;
        }
        cur.Next = new ListNode(key);
    }

    public void Remove(int key) {
        ListNode cur = set[key % set.Length];
        while (cur.Next != null) {
            if (cur.Next.Key == key) {
                cur.Next = cur.Next.Next;
                return;
            }
            cur = cur.Next;
        }
    }

    public bool Contains(int key) {
        ListNode cur = set[key % set.Length];
        while (cur.Next != null) {
            if (cur.Next.Key == key) return true;
            cur = cur.Next;
        }
        return false;
    }
}

type ListNode struct {
    key  int
    next *ListNode
}

type MyHashSet struct {
    set []*ListNode
}

func Constructor() MyHashSet {
    set := make([]*ListNode, 10000)
    for i := range set {
        set[i] = &ListNode{key: 0}
    }
    return MyHashSet{set: set}
}

func (this *MyHashSet) hash(key int) int {
    return key % len(this.set)
}

func (this *MyHashSet) Add(key int) {
    cur := this.set[this.hash(key)]
    for cur.next != nil {
        if cur.next.key == key {
            return
        }
        cur = cur.next
    }
    cur.next = &ListNode{key: key}
}

func (this *MyHashSet) Remove(key int) {
    cur := this.set[this.hash(key)]
    for cur.next != nil {
        if cur.next.key == key {
            cur.next = cur.next.next
            return
        }
        cur = cur.next
    }
}

func (this *MyHashSet) Contains(key int) bool {
    cur := this.set[this.hash(key)]
    for cur.next != nil {
        if cur.next.key == key {
            return true
        }
        cur = cur.next
    }
    return false
}

class ListNode(var key: Int, var next: ListNode? = null)

class MyHashSet() {
    private val set = Array(10000) { ListNode(0) }

    private fun hash(key: Int): Int = key % set.size

    fun add(key: Int) {
        var cur = set[hash(key)]
        while (cur.next != null) {
            if (cur.next!!.key == key) return
            cur = cur.next!!
        }
        cur.next = ListNode(key)
    }

    fun remove(key: Int) {
        var cur = set[hash(key)]
        while (cur.next != null) {
            if (cur.next!!.key == key) {
                cur.next = cur.next!!.next
                return
            }
            cur = cur.next!!
        }
    }

    fun contains(key: Int): Boolean {
        var cur = set[hash(key)]
        while (cur.next != null) {
            if (cur.next!!.key == key) return true
            cur = cur.next!!
        }
        return false
    }
}

class ListNode {
    var key: Int
    var next: ListNode?

    init(_ key: Int) {
        self.key = key
        self.next = nil
    }
}

class MyHashSet {
    private var set: [ListNode]

    init() {
        set = (0..<10000).map { _ in ListNode(0) }
    }

    private func hash(_ key: Int) -> Int {
        return key % set.count
    }

    func add(_ key: Int) {
        var cur = set[hash(key)]
        while cur.next != nil {
            if cur.next!.key == key {
                return
            }
            cur = cur.next!
        }
        cur.next = ListNode(key)
    }

    func remove(_ key: Int) {
        var cur = set[hash(key)]
        while cur.next != nil {
            if cur.next!.key == key {
                cur.next = cur.next!.next
                return
            }
            cur = cur.next!
        }
    }

    func contains(_ key: Int) -> Bool {
        var cur = set[hash(key)]
        while cur.next != nil {
            if cur.next!.key == key {
                return true
            }
            cur = cur.next!
        }
        return false
    }
}

Time & Space Complexity

Time complexity: $O(\frac{n}{k})$ for each function call.
Space complexity: $O(k + m)$

Where $n$ is the number of keys, $k$ is the size of the set ( $10000$ ) and $m$ is the number of unique keys.

4. Binary Search Tree

Intuition

Instead of linked lists for collision handling, we can use binary search trees (BSTs) in each bucket. This improves the worst-case time complexity from O(n/k) to O(log(n/k)) for each bucket, since BST operations are logarithmic in the number of nodes. The tradeoff is slightly more complex implementation.

Algorithm

Initialize an array of 10000 buckets, each containing an empty BST.
Define hash(key) as key % 10000.
For add(key): If the key is not already in the BST at hash(key), insert it using standard BST insertion.
For remove(key): Delete the key from the BST at hash(key) using standard BST deletion (finding in-order successor when needed).
For contains(key): Search the BST at hash(key) and return true if the key is found.

class TreeNode:
    def __init__(self, key):
        self.key = key
        self.left = None
        self.right = None

class BST:
    def __init__(self):
        self.root = None

    def insert(self, root, key):
        if not root:
            return TreeNode(key)
        if key < root.key:
            root.left = self.insert(root.left, key)
        elif key > root.key:
            root.right = self.insert(root.right, key)
        return root

    def delete(self, root, key):
        if not root:
            return None
        if key < root.key:
            root.left = self.delete(root.left, key)
        elif key > root.key:
            root.right = self.delete(root.right, key)
        else:
            if not root.left:
                return root.right
            if not root.right:
                return root.left
            temp = self.minValueNode(root.right)
            root.key = temp.key
            root.right = self.delete(root.right, temp.key)
        return root

    def minValueNode(self, root):
        while root.left:
            root = root.left
        return root

    def search(self, root, key):
        if not root:
            return False
        if key == root.key:
            return True
        elif key < root.key:
            return self.search(root.left, key)
        else:
            return self.search(root.right, key)

    def add(self, key):
        self.root = self.insert(self.root, key)

    def remove(self, key):
        self.root = self.delete(self.root, key)

    def contains(self, key):
        return self.search(self.root, key)

class MyHashSet:
    def __init__(self):
        self.size = 10000
        self.buckets = [BST() for _ in range(self.size)]

    def _hash(self, key):
        return key % self.size

    def add(self, key: int) -> None:
        idx = self._hash(key)
        if not self.contains(key):
            self.buckets[idx].add(key)

    def remove(self, key: int) -> None:
        idx = self._hash(key)
        self.buckets[idx].remove(key)

    def contains(self, key: int) -> bool:
        idx = self._hash(key)
        return self.buckets[idx].contains(key)

class TreeNode {
    int key;
    TreeNode left, right;

    TreeNode(int key) {
        this.key = key;
    }
}

class BST {
    private TreeNode root;

    private TreeNode insert(TreeNode node, int key) {
        if (node == null) return new TreeNode(key);
        if (key < node.key) node.left = insert(node.left, key);
        else if (key > node.key) node.right = insert(node.right, key);
        return node;
    }

    private TreeNode delete(TreeNode node, int key) {
        if (node == null) return null;
        if (key < node.key) node.left = delete(node.left, key);
        else if (key > node.key) node.right = delete(node.right, key);
        else {
            if (node.left == null) return node.right;
            if (node.right == null) return node.left;
            TreeNode temp = minValueNode(node.right);
            node.key = temp.key;
            node.right = delete(node.right, temp.key);
        }
        return node;
    }

    private TreeNode minValueNode(TreeNode node) {
        while (node.left != null) node = node.left;
        return node;
    }

    private boolean search(TreeNode node, int key) {
        if (node == null) return false;
        if (key == node.key) return true;
        return key < node.key ? search(node.left, key) : search(node.right, key);
    }

    public void add(int key) {
        root = insert(root, key);
    }

    public void remove(int key) {
        root = delete(root, key);
    }

    public boolean contains(int key) {
        return search(root, key);
    }
}

public class MyHashSet {
    private final int size = 10000;
    private BST[] buckets;

    public MyHashSet() {
        buckets = new BST[size];
        for (int i = 0; i < size; i++) {
            buckets[i] = new BST();
        }
    }

    private int hash(int key) {
        return key % size;
    }

    public void add(int key) {
        int idx = hash(key);
        if (!buckets[idx].contains(key)) {
            buckets[idx].add(key);
        }
    }

    public void remove(int key) {
        int idx = hash(key);
        buckets[idx].remove(key);
    }

    public boolean contains(int key) {
        int idx = hash(key);
        return buckets[idx].contains(key);
    }
}

class BST {
private:
    struct TreeNode {
        int key;
        TreeNode* left;
        TreeNode* right;
        TreeNode(int k) : key(k), left(nullptr), right(nullptr) {}
    };

    TreeNode* insert(TreeNode* root, int key) {
        if (!root) return new TreeNode(key);
        if (key < root->key)
            root->left = insert(root->left, key);
        else if (key > root->key)
            root->right = insert(root->right, key);
        return root;
    }

    TreeNode* deleteNode(TreeNode* root, int key) {
        if (!root) return nullptr;
        if (key < root->key)
            root->left = deleteNode(root->left, key);
        else if (key > root->key)
            root->right = deleteNode(root->right, key);
        else {
            if (!root->left) {
                TreeNode* temp = root->right;
                delete root;
                return temp;
            }
            if (!root->right) {
                TreeNode* temp = root->left;
                delete root;
                return temp;
            }
            TreeNode* temp = minValueNode(root->right);
            root->key = temp->key;
            root->right = deleteNode(root->right, temp->key);
        }
        return root;
    }

    TreeNode* minValueNode(TreeNode* root) {
        while (root->left) root = root->left;
        return root;
    }

    bool search(TreeNode* root, int key) {
        if (!root) return false;
        if (key == root->key) return true;
        return key < root->key ? search(root->left, key) : search(root->right, key);
    }

    TreeNode* root;

public:
    BST() : root(nullptr) {}

    void add(int key) {
        root = insert(root, key);
    }

    void remove(int key) {
        root = deleteNode(root, key);
    }

    bool contains(int key) {
        return search(root, key);
    }
};

class MyHashSet {
private:
    const int size = 10000;
    vector<BST> buckets;

    int hash(int key) {
        return key % size;
    }

public:
    MyHashSet() : buckets(size) {}

    void add(int key) {
        int idx = hash(key);
        if (!contains(key)) {
            buckets[idx].add(key);
        }
    }

    void remove(int key) {
        int idx = hash(key);
        buckets[idx].remove(key);
    }

    bool contains(int key) {
        int idx = hash(key);
        return buckets[idx].contains(key);
    }
};

class TreeNode {
    /**
     * @param {number} key
     */
    constructor(key) {
        this.key = key;
        this.left = null;
        this.right = null;
    }
}

class BST {
    constructor() {
        this.root = null;
    }

    /**
     * @param {number} key
     * @return {void}
     */
    add(key) {
        this.root = this._insert(this.root, key);
    }

    /**
     * @param {TreeNode} node
     * @param {number} key
     * @return {TreeNode}
     */
    _insert(node, key) {
        if (!node) return new TreeNode(key);
        if (key < node.key) node.left = this._insert(node.left, key);
        else if (key > node.key) node.right = this._insert(node.right, key);
        return node;
    }

    /**
     * @param {number} key
     * @return {void}
     */
    remove(key) {
        this.root = this._deleteNode(this.root, key);
    }

    /**
     * @param {TreeNode} node
     * @param {number} key
     * @return {TreeNode}
     */
    _deleteNode(node, key) {
        if (!node) return null;
        if (key < node.key) node.left = this._deleteNode(node.left, key);
        else if (key > node.key) node.right = this._deleteNode(node.right, key);
        else {
            if (!node.left) return node.right;
            if (!node.right) return node.left;
            let minNode = this._minValueNode(node.right);
            node.key = minNode.key;
            node.right = this._deleteNode(node.right, minNode.key);
        }
        return node;
    }

    /**
     * @param {TreeNode} node
     * @return {TreeNode}
     */
    _minValueNode(node) {
        while (node.left) node = node.left;
        return node;
    }

    /**
     * @param {number} key
     * @return {boolean}
     */
    contains(key) {
        return this._search(this.root, key);
    }

    /**
     * @param {TreeNode} node
     * @param {number} key
     * @return {boolean}
     */
    _search(node, key) {
        if (!node) return false;
        if (key === node.key) return true;
        if (key < node.key) return this._search(node.left, key);
        return this._search(node.right, key);
    }
}

class MyHashSet {
    constructor() {
        this.size = 10000;
        this.buckets = Array.from({ length: this.size }, () => new BST());
    }

    _hash(key) {
        return key % this.size;
    }

    /**
     * @param {number} key
     * @return {void}
     */
    add(key) {
        const idx = this._hash(key);
        if (!this.buckets[idx].contains(key)) {
            this.buckets[idx].add(key);
        }
    }

    /**
     * @param {number} key
     * @return {void}
     */
    remove(key) {
        const idx = this._hash(key);
        this.buckets[idx].remove(key);
    }

    /**
     * @param {number} key
     * @return {boolean}
     */
    contains(key) {
        const idx = this._hash(key);
        return this.buckets[idx].contains(key);
    }
}

public class TreeNode {
    public int Key;
    public TreeNode Left;
    public TreeNode Right;

    public TreeNode(int key) {
        Key = key;
        Left = null;
        Right = null;
    }
}

public class BST {
    public TreeNode Root;

    public TreeNode Insert(TreeNode root, int key) {
        if (root == null) return new TreeNode(key);
        if (key < root.Key)
            root.Left = Insert(root.Left, key);
        else if (key > root.Key)
            root.Right = Insert(root.Right, key);
        return root;
    }

    public TreeNode Delete(TreeNode root, int key) {
        if (root == null) return null;
        if (key < root.Key)
            root.Left = Delete(root.Left, key);
        else if (key > root.Key)
            root.Right = Delete(root.Right, key);
        else {
            if (root.Left == null) return root.Right;
            if (root.Right == null) return root.Left;
            TreeNode temp = MinValueNode(root.Right);
            root.Key = temp.Key;
            root.Right = Delete(root.Right, temp.Key);
        }
        return root;
    }

    private TreeNode MinValueNode(TreeNode root) {
        while (root.Left != null)
            root = root.Left;
        return root;
    }

    public bool Search(TreeNode root, int key) {
        if (root == null) return false;
        if (key == root.Key) return true;
        if (key < root.Key) return Search(root.Left, key);
        return Search(root.Right, key);
    }

    public void Add(int key) {
        Root = Insert(Root, key);
    }

    public void Remove(int key) {
        Root = Delete(Root, key);
    }

    public bool Contains(int key) {
        return Search(Root, key);
    }
}

public class MyHashSet {
    private const int Size = 10000;
    private BST[] buckets;

    public MyHashSet() {
        buckets = new BST[Size];
        for (int i = 0; i < Size; i++) {
            buckets[i] = new BST();
        }
    }

    private int Hash(int key) {
        return key % Size;
    }

    public void Add(int key) {
        int idx = Hash(key);
        if (!Contains(key)) {
            buckets[idx].Add(key);
        }
    }

    public void Remove(int key) {
        int idx = Hash(key);
        buckets[idx].Remove(key);
    }

    public bool Contains(int key) {
        int idx = Hash(key);
        return buckets[idx].Contains(key);
    }
}

type TreeNode struct {
    key   int
    left  *TreeNode
    right *TreeNode
}

type BST struct {
    root *TreeNode
}

func (b *BST) insert(node *TreeNode, key int) *TreeNode {
    if node == nil {
        return &TreeNode{key: key}
    }
    if key < node.key {
        node.left = b.insert(node.left, key)
    } else if key > node.key {
        node.right = b.insert(node.right, key)
    }
    return node
}

func (b *BST) deleteNode(node *TreeNode, key int) *TreeNode {
    if node == nil {
        return nil
    }
    if key < node.key {
        node.left = b.deleteNode(node.left, key)
    } else if key > node.key {
        node.right = b.deleteNode(node.right, key)
    } else {
        if node.left == nil {
            return node.right
        }
        if node.right == nil {
            return node.left
        }
        minNode := b.minValueNode(node.right)
        node.key = minNode.key
        node.right = b.deleteNode(node.right, minNode.key)
    }
    return node
}

func (b *BST) minValueNode(node *TreeNode) *TreeNode {
    for node.left != nil {
        node = node.left
    }
    return node
}

func (b *BST) search(node *TreeNode, key int) bool {
    if node == nil {
        return false
    }
    if key == node.key {
        return true
    }
    if key < node.key {
        return b.search(node.left, key)
    }
    return b.search(node.right, key)
}

func (b *BST) Add(key int) {
    b.root = b.insert(b.root, key)
}

func (b *BST) Remove(key int) {
    b.root = b.deleteNode(b.root, key)
}

func (b *BST) Contains(key int) bool {
    return b.search(b.root, key)
}

type MyHashSet struct {
    size    int
    buckets []*BST
}

func Constructor() MyHashSet {
    size := 10000
    buckets := make([]*BST, size)
    for i := range buckets {
        buckets[i] = &BST{}
    }
    return MyHashSet{size: size, buckets: buckets}
}

func (this *MyHashSet) hash(key int) int {
    return key % this.size
}

func (this *MyHashSet) Add(key int) {
    idx := this.hash(key)
    if !this.Contains(key) {
        this.buckets[idx].Add(key)
    }
}

func (this *MyHashSet) Remove(key int) {
    idx := this.hash(key)
    this.buckets[idx].Remove(key)
}

func (this *MyHashSet) Contains(key int) bool {
    idx := this.hash(key)
    return this.buckets[idx].Contains(key)
}

class TreeNode(var key: Int) {
    var left: TreeNode? = null
    var right: TreeNode? = null
}

class BST {
    var root: TreeNode? = null

    private fun insert(node: TreeNode?, key: Int): TreeNode {
        if (node == null) return TreeNode(key)
        if (key < node.key) node.left = insert(node.left, key)
        else if (key > node.key) node.right = insert(node.right, key)
        return node
    }

    private fun delete(node: TreeNode?, key: Int): TreeNode? {
        if (node == null) return null
        if (key < node.key) node.left = delete(node.left, key)
        else if (key > node.key) node.right = delete(node.right, key)
        else {
            if (node.left == null) return node.right
            if (node.right == null) return node.left
            val minNode = minValueNode(node.right!!)
            node.key = minNode.key
            node.right = delete(node.right, minNode.key)
        }
        return node
    }

    private fun minValueNode(node: TreeNode): TreeNode {
        var cur = node
        while (cur.left != null) cur = cur.left!!
        return cur
    }

    private fun search(node: TreeNode?, key: Int): Boolean {
        if (node == null) return false
        if (key == node.key) return true
        return if (key < node.key) search(node.left, key) else search(node.right, key)
    }

    fun add(key: Int) { root = insert(root, key) }
    fun remove(key: Int) { root = delete(root, key) }
    fun contains(key: Int): Boolean = search(root, key)
}

class MyHashSet() {
    private val size = 10000
    private val buckets = Array(size) { BST() }

    private fun hash(key: Int): Int = key % size

    fun add(key: Int) {
        val idx = hash(key)
        if (!contains(key)) buckets[idx].add(key)
    }

    fun remove(key: Int) {
        buckets[hash(key)].remove(key)
    }

    fun contains(key: Int): Boolean {
        return buckets[hash(key)].contains(key)
    }
}

class TreeNode {
    var key: Int
    var left: TreeNode?
    var right: TreeNode?
    init(_ key: Int) { self.key = key }
}

class BST {
    var root: TreeNode?

    func insert(_ node: TreeNode?, _ key: Int) -> TreeNode {
        guard let node = node else { return TreeNode(key) }
        if key < node.key { node.left = insert(node.left, key) }
        else if key > node.key { node.right = insert(node.right, key) }
        return node
    }

    func delete(_ node: TreeNode?, _ key: Int) -> TreeNode? {
        guard let node = node else { return nil }
        if key < node.key { node.left = delete(node.left, key) }
        else if key > node.key { node.right = delete(node.right, key) }
        else {
            if node.left == nil { return node.right }
            if node.right == nil { return node.left }
            let minNode = minValueNode(node.right!)
            node.key = minNode.key
            node.right = delete(node.right, minNode.key)
        }
        return node
    }

    func minValueNode(_ node: TreeNode) -> TreeNode {
        var cur = node
        while cur.left != nil { cur = cur.left! }
        return cur
    }

    func search(_ node: TreeNode?, _ key: Int) -> Bool {
        guard let node = node else { return false }
        if key == node.key { return true }
        return key < node.key ? search(node.left, key) : search(node.right, key)
    }

    func add(_ key: Int) { root = insert(root, key) }
    func remove(_ key: Int) { root = delete(root, key) }
    func contains(_ key: Int) -> Bool { return search(root, key) }
}

class MyHashSet {
    private let size = 10000
    private var buckets: [BST]

    init() {
        buckets = (0..<size).map { _ in BST() }
    }

    private func hash(_ key: Int) -> Int { return key % size }

    func add(_ key: Int) {
        let idx = hash(key)
        if !contains(key) { buckets[idx].add(key) }
    }

    func remove(_ key: Int) {
        buckets[hash(key)].remove(key)
    }

    func contains(_ key: Int) -> Bool {
        return buckets[hash(key)].contains(key)
    }
}

Time & Space Complexity

Time complexity: $O(\log (\frac{n}{k}))$ in average case, $O(\frac{n}{k})$ in worst case for each function call.
Space complexity: $O(k + m)$

Where $n$ is the number of keys, $k$ is the size of the set ( $10000$ ) and $m$ is the number of unique keys.

5. Bit Manipulation

Intuition

We can compress the boolean array approach by using individual bits instead of booleans. Each integer stores 32 bits, so we need only about 31251 integers to cover 1000000+ keys. We use bit operations to set, clear, and check individual bits. This reduces memory usage by a factor of 32 compared to a boolean array.

Algorithm

Initialize an integer array of size 31251 (since 31251 * 32 = 1000032 covers all keys).
Define getMask(key) as 1 << (key % 32) to create a bitmask for the key's position within its integer.
For add(key): Set the bit using set[key / 32] |= getMask(key).
For remove(key): If the key exists, toggle the bit using set[key / 32] ^= getMask(key).
For contains(key): Return true if set[key / 32] & getMask(key) is non-zero.

class MyHashSet:

    def __init__(self):
        # key is in the range [1, 1000000]
        # 31251 * 32 = 1000032
        self.set = [0] * 31251

    def add(self, key: int) -> None:
        self.set[key // 32] |= self.getMask(key)

    def remove(self, key: int) -> None:
        if self.contains(key):
            self.set[key // 32] ^= self.getMask(key)

    def contains(self, key: int) -> bool:
        return self.set[key // 32] & self.getMask(key) != 0

    def getMask(self, key: int) -> int:
        return 1 << (key % 32)

public class MyHashSet {
    private int[] set;

    public MyHashSet() {
        // key is in the range [1, 1000000]
        // 31251 * 32 = 1000032
        set = new int[31251];
    }

    public void add(int key) {
        set[key / 32] |= getMask(key);
    }

    public void remove(int key) {
        if (contains(key)) {
            set[key / 32] ^= getMask(key);
        }
    }

    public boolean contains(int key) {
        return (set[key / 32] & getMask(key)) != 0;
    }

    private int getMask(int key) {
        return 1 << (key % 32);
    }
}

class MyHashSet {
private:
    int set[31251];

    int getMask(int key) {
        return 1 << (key % 32);
    }

public:
    MyHashSet() {
        // key is in the range [1, 1000000]
        // 31251 * 32 = 1000032
        memset(set, 0, sizeof(set));
    }

    void add(int key) {
        set[key / 32] |= getMask(key);
    }

    void remove(int key) {
        if (contains(key)) {
            set[key / 32] ^= getMask(key);
        }
    }

    bool contains(int key) {
        return (set[key / 32] & getMask(key)) != 0;
    }
};

class MyHashSet {
    constructor() {
        // key is in the range [1, 1000000]
        // 31251 * 32 = 1000032
        this.set = new Array(31251).fill(0);
    }

    /**
     * @param {number} key
     * @return {void}
     */
    add(key) {
        this.set[Math.floor(key / 32)] |= this.getMask(key);
    }

    /**
     * @param {number} key
     * @return {void}
     */
    remove(key) {
        if (this.contains(key)) {
            this.set[Math.floor(key / 32)] ^= this.getMask(key);
        }
    }

    /**
     * @param {number} key
     * @return {boolean}
     */
    contains(key) {
        return (this.set[Math.floor(key / 32)] & this.getMask(key)) !== 0;
    }

    /**
     * @param {number} key
     * @return {number}
     */
    getMask(key) {
        return 1 << key % 32;
    }
}

public class MyHashSet {
    private int[] set;

    public MyHashSet() {
        // key is in the range [1, 1000000]
        // 31251 * 32 = 1000032
        set = new int[31251];
    }

    public void Add(int key) {
        set[key / 32] |= GetMask(key);
    }

    public void Remove(int key) {
        if (Contains(key)) {
            set[key / 32] ^= GetMask(key);
        }
    }

    public bool Contains(int key) {
        return (set[key / 32] & GetMask(key)) != 0;
    }

    private int GetMask(int key) {
        return 1 << (key % 32);
    }
}

type MyHashSet struct {
    set []int
}

func Constructor() MyHashSet {
    // key is in the range [1, 1000000]
    // 31251 * 32 = 1000032
    return MyHashSet{set: make([]int, 31251)}
}

func (this *MyHashSet) getMask(key int) int {
    return 1 << (key % 32)
}

func (this *MyHashSet) Add(key int) {
    this.set[key/32] |= this.getMask(key)
}

func (this *MyHashSet) Remove(key int) {
    if this.Contains(key) {
        this.set[key/32] ^= this.getMask(key)
    }
}

func (this *MyHashSet) Contains(key int) bool {
    return (this.set[key/32] & this.getMask(key)) != 0
}

class MyHashSet() {
    // key is in the range [1, 1000000]
    // 31251 * 32 = 1000032
    private val set = IntArray(31251)

    private fun getMask(key: Int): Int = 1 shl (key % 32)

    fun add(key: Int) {
        set[key / 32] = set[key / 32] or getMask(key)
    }

    fun remove(key: Int) {
        if (contains(key)) {
            set[key / 32] = set[key / 32] xor getMask(key)
        }
    }

    fun contains(key: Int): Boolean {
        return (set[key / 32] and getMask(key)) != 0
    }
}

class MyHashSet {
    // key is in the range [1, 1000000]
    // 31251 * 32 = 1000032
    private var set: [Int]

    init() {
        set = [Int](repeating: 0, count: 31251)
    }

    private func getMask(_ key: Int) -> Int {
        return 1 << (key % 32)
    }

    func add(_ key: Int) {
        set[key / 32] |= getMask(key)
    }

    func remove(_ key: Int) {
        if contains(key) {
            set[key / 32] ^= getMask(key)
        }
    }

    func contains(_ key: Int) -> Bool {
        return (set[key / 32] & getMask(key)) != 0
    }
}

Time & Space Complexity

Time complexity: $O(1)$ for each function call.
Space complexity: $O(k)$

Where $k$ is the size of the set $(31251)$ .

Common Pitfalls

Adding Duplicate Keys

The add() operation should be idempotent - adding an existing key should have no effect. A common mistake is not checking for duplicates before insertion.

# Wrong - allows duplicates
def add(self, key: int) -> None:
    cur = self.set[key % len(self.set)]
    while cur.next:
        cur = cur.next
    cur.next = ListNode(key)  # Always adds, even if exists!

# Correct - check for existence first
def add(self, key: int) -> None:
    cur = self.set[key % len(self.set)]
    while cur.next:
        if cur.next.key == key:
            return  # Already exists
        cur = cur.next
    cur.next = ListNode(key)

Using XOR for Remove Without Checking Existence

In the bit manipulation approach, using XOR to remove a key that doesn't exist will incorrectly add it instead.

# Wrong - XOR toggles the bit regardless
def remove(self, key: int) -> None:
    self.set[key // 32] ^= self.getMask(key)  # Adds if not present!

# Correct - only toggle if present
def remove(self, key: int) -> None:
    if self.contains(key):
        self.set[key // 32] ^= self.getMask(key)