438. Find All Anagrams in a String - Explanation

Prerequisites

Before attempting this problem, you should be comfortable with:

Sliding Window Technique - Required for efficiently processing substrings of fixed length without recomputing from scratch
Hash Maps / Frequency Arrays - Used to count and compare character frequencies between strings
String Manipulation - Understanding how to extract substrings and compare character patterns

1. Brute Force

Intuition

An anagram is simply a rearrangement of characters. Two strings are anagrams if they contain the same characters with the same frequencies. The most direct way to check this is to sort both strings and compare them. We can slide through every substring of s that has the same length as p, sort it, and check if it matches the sorted version of p.

Algorithm

Sort the pattern string p.
For each starting index i from 0 to len(s) - len(p):
- Extract the substring of length len(p) starting at i.
- Sort this substring.
- If it equals the sorted pattern, add i to the result list.
Return the result list.

class Solution:
    def findAnagrams(self, s: str, p: str) -> List[int]:
        n, m = len(s), len(p)
        p = sorted(p)
        res = []
        for i in range(n - m + 1):
            sub = sorted(s[i : i + m])
            if sub == p:
                res.append(i)
        return res

public class Solution {
    public List<Integer> findAnagrams(String s, String p) {
        int n = s.length(), m = p.length();
        List<Integer> res = new ArrayList<>();
        char[] pArr = p.toCharArray();
        Arrays.sort(pArr);
        String sortedP = new String(pArr);

        for (int i = 0; i <= n - m; i++) {
            char[] subArr = s.substring(i, i + m).toCharArray();
            Arrays.sort(subArr);
            if (new String(subArr).equals(sortedP)) {
                res.add(i);
            }
        }
        return res;
    }
}

class Solution {
public:
    vector<int> findAnagrams(string s, string p) {
        int n = s.size(), m = p.size();
        vector<int> res;
        sort(p.begin(), p.end());

        for (int i = 0; i <= n - m; i++) {
            string sub = s.substr(i, m);
            sort(sub.begin(), sub.end());
            if (sub == p) {
                res.push_back(i);
            }
        }
        return res;
    }
};

class Solution {
    /**
     * @param {string} s
     * @param {string} p
     * @return {number[]}
     */
    findAnagrams(s, p) {
        const n = s.length,
            m = p.length;
        const res = [];
        const sortedP = p.split('').sort().join('');

        for (let i = 0; i <= n - m; i++) {
            const sub = s
                .substring(i, i + m)
                .split('')
                .sort()
                .join('');
            if (sub === sortedP) {
                res.push(i);
            }
        }
        return res;
    }
}

public class Solution {
    public IList<int> FindAnagrams(string s, string p) {
        int n = s.Length, m = p.Length;
        List<int> res = new List<int>();
        char[] pArr = p.ToCharArray();
        Array.Sort(pArr);
        string sortedP = new string(pArr);

        for (int i = 0; i <= n - m; i++) {
            char[] subArr = s.Substring(i, m).ToCharArray();
            Array.Sort(subArr);
            if (new string(subArr) == sortedP) {
                res.Add(i);
            }
        }
        return res;
    }
}

class Solution {
    fun findAnagrams(s: String, p: String): List<Int> {
        val n = s.length
        val m = p.length
        val res = mutableListOf<Int>()
        val sortedP = p.toCharArray().sorted().joinToString("")

        for (i in 0..n - m) {
            val sub = s.substring(i, i + m).toCharArray().sorted().joinToString("")
            if (sub == sortedP) {
                res.add(i)
            }
        }
        return res
    }
}

class Solution {
    func findAnagrams(_ s: String, _ p: String) -> [Int] {
        let n = s.count, m = p.count
        var res = [Int]()
        let sortedP = String(p.sorted())
        let sArr = Array(s)

        for i in 0...(n - m) {
            let sub = String(sArr[i..<(i + m)].sorted())
            if sub == sortedP {
                res.append(i)
            }
        }
        return res
    }
}

Time & Space Complexity

Time complexity: $O(n * m \log m)$
Space complexity: $O(m)$

Where $n$ is the length of the string $s$ and $m$ is the length of the string $p$ .

2. Prefix Count + Sliding Window

Intuition

Instead of sorting substrings repeatedly, we can precompute character frequencies using prefix sums. For each position in s, we maintain a cumulative count of each character seen so far. To get the character frequencies for any window, we subtract the prefix count at the start from the prefix count at the end. If the window's character counts match p's counts, we found an anagram.

Algorithm

Build a frequency array pCount for the pattern p.
Build a 2D prefix array where prefix[i] stores the cumulative character counts for s[0..i-1].
Slide a window of size len(p) across s:
- For each window position, compute character frequencies using prefix[j+1] - prefix[i].
- If all 26 character counts match pCount, add the starting index to the result.
Return the result list.

class Solution:
    def findAnagrams(self, s: str, p: str) -> List[int]:
        n, m = len(s), len(p)
        if m > n:
            return []
        pCount = [0] * 26
        for c in p:
            pCount[ord(c) - ord('a')] += 1

        prefix = [[0] * 26 for _ in range(n + 1)]
        for i in range(1, n + 1):
            for j in range(26):
                prefix[i][j] = prefix[i - 1][j]
            prefix[i][ord(s[i - 1]) - ord('a')] += 1

        i, j = 0, m - 1
        res = []
        while j < n:
            isValid = True
            for c in range(26):
                if prefix[j + 1][c] - prefix[i][c] != pCount[c]:
                    isValid = False
                    break
            if isValid:
                res.append(i)
            i += 1
            j += 1

        return res

public class Solution {
    public List<Integer> findAnagrams(String s, String p) {
        int n = s.length(), m = p.length();
        if (m > n) return new ArrayList<>();

        int[] pCount = new int[26];
        for (char c : p.toCharArray()) {
            pCount[c - 'a']++;
        }

        int[][] prefix = new int[n + 1][26];
        for (int i = 1; i <= n; i++) {
            System.arraycopy(prefix[i - 1], 0, prefix[i], 0, 26);
            prefix[i][s.charAt(i - 1) - 'a']++;
        }

        List<Integer> res = new ArrayList<>();
        int i = 0, j = m - 1;
        while (j < n) {
            boolean isValid = true;
            for (int c = 0; c < 26; c++) {
                if (prefix[j + 1][c] - prefix[i][c] != pCount[c]) {
                    isValid = false;
                    break;
                }
            }
            if (isValid) res.add(i);
            i++;
            j++;
        }

        return res;
    }
}

class Solution {
public:
    vector<int> findAnagrams(string s, string p) {
        int n = s.size(), m = p.size();
        if (m > n) return {};

        vector<int> pCount(26, 0);
        for (char c : p) pCount[c - 'a']++;

        vector<vector<int>> prefix(n + 1, vector<int>(26, 0));
        for (int i = 1; i <= n; i++) {
            prefix[i] = prefix[i - 1];
            prefix[i][s[i - 1] - 'a']++;
        }

        vector<int> res;
        int i = 0, j = m - 1;
        while (j < n) {
            bool isValid = true;
            for (int c = 0; c < 26; c++) {
                if (prefix[j + 1][c] - prefix[i][c] != pCount[c]) {
                    isValid = false;
                    break;
                }
            }
            if (isValid) res.push_back(i);
            i++;
            j++;
        }

        return res;
    }
};

class Solution {
    /**
     * @param {string} s
     * @param {string} p
     * @return {number[]}
     */
    findAnagrams(s, p) {
        const n = s.length,
            m = p.length;
        if (m > n) return [];

        const pCount = Array(26).fill(0);
        for (const c of p) {
            pCount[c.charCodeAt(0) - 97]++;
        }

        const prefix = Array.from({ length: n + 1 }, () => Array(26).fill(0));
        for (let i = 1; i <= n; i++) {
            for (let j = 0; j < 26; j++) {
                prefix[i][j] = prefix[i - 1][j];
            }
            prefix[i][s.charCodeAt(i - 1) - 97]++;
        }

        const res = [];
        let i = 0,
            j = m - 1;
        while (j < n) {
            let isValid = true;
            for (let c = 0; c < 26; c++) {
                if (prefix[j + 1][c] - prefix[i][c] !== pCount[c]) {
                    isValid = false;
                    break;
                }
            }
            if (isValid) res.push(i);
            i++;
            j++;
        }

        return res;
    }
}

public class Solution {
    public IList<int> FindAnagrams(string s, string p) {
        int n = s.Length, m = p.Length;
        if (m > n) return new List<int>();

        int[] pCount = new int[26];
        foreach (char c in p) {
            pCount[c - 'a']++;
        }

        int[][] prefix = new int[n + 1][];
        for (int i = 0; i <= n; i++) {
            prefix[i] = new int[26];
        }
        for (int i = 1; i <= n; i++) {
            Array.Copy(prefix[i - 1], prefix[i], 26);
            prefix[i][s[i - 1] - 'a']++;
        }

        List<int> res = new List<int>();
        int left = 0, right = m - 1;
        while (right < n) {
            bool isValid = true;
            for (int c = 0; c < 26; c++) {
                if (prefix[right + 1][c] - prefix[left][c] != pCount[c]) {
                    isValid = false;
                    break;
                }
            }
            if (isValid) res.Add(left);
            left++;
            right++;
        }

        return res;
    }
}

func findAnagrams(s string, p string) []int {
    n, m := len(s), len(p)
    if m > n {
        return []int{}
    }

    pCount := make([]int, 26)
    for _, c := range p {
        pCount[c-'a']++
    }

    prefix := make([][]int, n+1)
    for i := range prefix {
        prefix[i] = make([]int, 26)
    }
    for i := 1; i <= n; i++ {
        copy(prefix[i], prefix[i-1])
        prefix[i][s[i-1]-'a']++
    }

    res := []int{}
    i, j := 0, m-1
    for j < n {
        isValid := true
        for c := 0; c < 26; c++ {
            if prefix[j+1][c]-prefix[i][c] != pCount[c] {
                isValid = false
                break
            }
        }
        if isValid {
            res = append(res, i)
        }
        i++
        j++
    }

    return res
}

class Solution {
    fun findAnagrams(s: String, p: String): List<Int> {
        val n = s.length
        val m = p.length
        if (m > n) return emptyList()

        val pCount = IntArray(26)
        for (c in p) {
            pCount[c - 'a']++
        }

        val prefix = Array(n + 1) { IntArray(26) }
        for (i in 1..n) {
            prefix[i - 1].copyInto(prefix[i])
            prefix[i][s[i - 1] - 'a']++
        }

        val res = mutableListOf<Int>()
        var i = 0
        var j = m - 1
        while (j < n) {
            var isValid = true
            for (c in 0 until 26) {
                if (prefix[j + 1][c] - prefix[i][c] != pCount[c]) {
                    isValid = false
                    break
                }
            }
            if (isValid) res.add(i)
            i++
            j++
        }

        return res
    }
}

class Solution {
    func findAnagrams(_ s: String, _ p: String) -> [Int] {
        let n = s.count, m = p.count
        if m > n { return [] }

        var pCount = [Int](repeating: 0, count: 26)
        for c in p {
            pCount[Int(c.asciiValue!) - 97] += 1
        }

        let sArr = Array(s)
        var prefix = [[Int]](repeating: [Int](repeating: 0, count: 26), count: n + 1)
        for i in 1...n {
            prefix[i] = prefix[i - 1]
            prefix[i][Int(sArr[i - 1].asciiValue!) - 97] += 1
        }

        var res = [Int]()
        var i = 0, j = m - 1
        while j < n {
            var isValid = true
            for c in 0..<26 {
                if prefix[j + 1][c] - prefix[i][c] != pCount[c] {
                    isValid = false
                    break
                }
            }
            if isValid { res.append(i) }
            i += 1
            j += 1
        }

        return res
    }
}

Time & Space Complexity

Time complexity: $O(n + m)$
Space complexity: $O(n)$

Where $n$ is the length of the string $s$ and $m$ is the length of the string $p$ .

3. Sliding Window

Intuition

We can avoid rebuilding frequency counts from scratch by using a sliding window. Start by counting characters in the first window of size len(p). As we slide the window one position to the right, we add the new character entering the window and remove the character leaving it. After each slide, we compare the window's character counts with the pattern's counts.

Algorithm

Build frequency arrays pCount and sCount for p and the first len(p) characters of s.
If they match, add index 0 to the result.
Slide the window from position len(p) to the end of s:
- Add the new character at the right end to sCount.
- Remove the character at the left end from sCount.
- Move the left pointer forward.
- If sCount equals pCount, add the new left index to the result.
Return the result list.

class Solution:
    def findAnagrams(self, s: str, p: str) -> List[int]:
        if len(p) > len(s): return []
        pCount, sCount = {}, {}
        for i in range(len(p)):
            pCount[p[i]] = 1 + pCount.get(p[i], 0)
            sCount[s[i]] = 1+ sCount.get(s[i], 0)

        res = [0] if sCount == pCount else []
        l = 0
        for r in range(len(p), len(s)):
            sCount[s[r]] = 1+ sCount.get(s[r], 0)
            sCount[s[l]] -= 1

            if sCount[s[l]] == 0:
                sCount.pop(s[l])
            l += 1
            if sCount == pCount:
                res.append(l)
        return res

public class Solution {
    public List<Integer> findAnagrams(String s, String p) {
        if (p.length() > s.length()) return new ArrayList<>();

        int[] pCount = new int[26];
        int[] sCount = new int[26];

        for (char c : p.toCharArray()) {
            pCount[c - 'a']++;
        }
        for (int i = 0; i < p.length(); i++) {
            sCount[s.charAt(i) - 'a']++;
        }

        List<Integer> res = new ArrayList<>();
        if (Arrays.equals(pCount, sCount)) res.add(0);

        int l = 0;
        for (int r = p.length(); r < s.length(); r++) {
            sCount[s.charAt(r) - 'a']++;
            sCount[s.charAt(l) - 'a']--;
            l++;
            if (Arrays.equals(pCount, sCount)) {
                res.add(l);
            }
        }

        return res;
    }
}

class Solution {
public:
    vector<int> findAnagrams(string s, string p) {
        if (p.size() > s.size()) return {};

        vector<int> pCount(26, 0), sCount(26, 0);
        for (char c : p) {
            pCount[c - 'a']++;
        }
        for (int i = 0; i < p.size(); i++) {
            sCount[s[i] - 'a']++;
        }

        vector<int> res;
        if (pCount == sCount) res.push_back(0);

        int l = 0;
        for (int r = p.size(); r < s.size(); r++) {
            sCount[s[r] - 'a']++;
            sCount[s[l] - 'a']--;
            l++;
            if (pCount == sCount) {
                res.push_back(l);
            }
        }

        return res;
    }
};

class Solution {
    /**
     * @param {string} s
     * @param {string} p
     * @return {number[]}
     */
    findAnagrams(s, p) {
        if (p.length > s.length) return [];

        const pCount = new Array(26).fill(0);
        const sCount = new Array(26).fill(0);

        for (const char of p) {
            pCount[char.charCodeAt(0) - 97]++;
        }
        for (let i = 0; i < p.length; i++) {
            sCount[s.charCodeAt(i) - 97]++;
        }

        const res = [];
        if (pCount.toString() === sCount.toString()) res.push(0);

        let l = 0;
        for (let r = p.length; r < s.length; r++) {
            sCount[s.charCodeAt(r) - 97]++;
            sCount[s.charCodeAt(l) - 97]--;
            l++;
            if (pCount.toString() === sCount.toString()) {
                res.push(l);
            }
        }

        return res;
    }
}

public class Solution {
    public IList<int> FindAnagrams(string s, string p) {
        if (p.Length > s.Length) return new List<int>();

        int[] pCount = new int[26];
        int[] sCount = new int[26];

        foreach (char c in p) {
            pCount[c - 'a']++;
        }
        for (int i = 0; i < p.Length; i++) {
            sCount[s[i] - 'a']++;
        }

        List<int> res = new List<int>();
        if (pCount.SequenceEqual(sCount)) res.Add(0);

        int l = 0;
        for (int r = p.Length; r < s.Length; r++) {
            sCount[s[r] - 'a']++;
            sCount[s[l] - 'a']--;
            l++;
            if (pCount.SequenceEqual(sCount)) {
                res.Add(l);
            }
        }

        return res;
    }
}

func findAnagrams(s string, p string) []int {
    if len(p) > len(s) {
        return []int{}
    }

    pCount := make([]int, 26)
    sCount := make([]int, 26)

    for _, c := range p {
        pCount[c-'a']++
    }
    for i := 0; i < len(p); i++ {
        sCount[s[i]-'a']++
    }

    equal := func() bool {
        for i := 0; i < 26; i++ {
            if pCount[i] != sCount[i] {
                return false
            }
        }
        return true
    }

    res := []int{}
    if equal() {
        res = append(res, 0)
    }

    l := 0
    for r := len(p); r < len(s); r++ {
        sCount[s[r]-'a']++
        sCount[s[l]-'a']--
        l++
        if equal() {
            res = append(res, l)
        }
    }

    return res
}

class Solution {
    fun findAnagrams(s: String, p: String): List<Int> {
        if (p.length > s.length) return emptyList()

        val pCount = IntArray(26)
        val sCount = IntArray(26)

        for (c in p) {
            pCount[c - 'a']++
        }
        for (i in p.indices) {
            sCount[s[i] - 'a']++
        }

        val res = mutableListOf<Int>()
        if (pCount.contentEquals(sCount)) res.add(0)

        var l = 0
        for (r in p.length until s.length) {
            sCount[s[r] - 'a']++
            sCount[s[l] - 'a']--
            l++
            if (pCount.contentEquals(sCount)) {
                res.add(l)
            }
        }

        return res
    }
}

class Solution {
    func findAnagrams(_ s: String, _ p: String) -> [Int] {
        if p.count > s.count { return [] }

        var pCount = [Int](repeating: 0, count: 26)
        var sCount = [Int](repeating: 0, count: 26)
        let sArr = Array(s)
        let pArr = Array(p)

        for c in pArr {
            pCount[Int(c.asciiValue!) - 97] += 1
        }
        for i in 0..<pArr.count {
            sCount[Int(sArr[i].asciiValue!) - 97] += 1
        }

        var res = [Int]()
        if pCount == sCount { res.append(0) }

        var l = 0
        for r in pArr.count..<sArr.count {
            sCount[Int(sArr[r].asciiValue!) - 97] += 1
            sCount[Int(sArr[l].asciiValue!) - 97] -= 1
            l += 1
            if pCount == sCount {
                res.append(l)
            }
        }

        return res
    }
}

Time & Space Complexity

Time complexity: $O(n + m)$
Space complexity: $O(1)$ since we have at most $26$ different characters.

Where $n$ is the length of the string $s$ and $m$ is the length of the string $p$ .

4. Sliding Window (Optimal)

Intuition

Comparing two arrays of 26 elements after every slide takes extra time. We can optimize by tracking how many of the 26 character counts currently match between the window and the pattern. When we add or remove a character, we only update the match count for that specific character. If all 26 counts match, we found an anagram.

Algorithm

Build frequency arrays pCount and sCount for the initial window.
Count how many of the 26 characters have matching frequencies (initialize match).
If match == 26, add index 0 to the result.
Slide the window across s:
- For the character leaving the window: update its count and adjust match accordingly.
- For the character entering the window: update its count and adjust match accordingly.
- If match == 26, add the current left index to the result.
Return the result list.

class Solution:
    def findAnagrams(self, s: str, p: str) -> List[int]:
        n, m = len(s), len(p)
        if m > n:
            return []

        pCount = [0] * 26
        sCount = [0] * 26
        for i in range(m):
            pCount[ord(p[i]) - ord('a')] += 1
            sCount[ord(s[i]) - ord('a')] += 1

        match = sum(1 for i in range(26) if pCount[i] == sCount[i])
        res = []
        if match == 26:
            res.append(0)

        l = 0
        for r in range(m, n):
            c = ord(s[l]) - ord('a')
            if sCount[c] == pCount[c]:
                match -= 1
            sCount[c] -= 1
            l += 1
            if sCount[c] == pCount[c]:
                match += 1

            c = ord(s[r]) - ord('a')
            if sCount[c] == pCount[c]:
                match -= 1
            sCount[c] += 1
            if sCount[c] == pCount[c]:
                match += 1

            if match == 26:
                res.append(l)

        return res

public class Solution {
    public List<Integer> findAnagrams(String s, String p) {
        int n = s.length(), m = p.length();
        if (m > n) return new ArrayList<>();

        int[] pCount = new int[26], sCount = new int[26];
        for (int i = 0; i < m; i++) {
            pCount[p.charAt(i) - 'a']++;
            sCount[s.charAt(i) - 'a']++;
        }

        int match = 0;
        for (int i = 0; i < 26; i++) {
            if (pCount[i] == sCount[i]) match++;
        }

        List<Integer> res = new ArrayList<>();
        if (match == 26) res.add(0);

        int l = 0;
        for (int r = m; r < n; r++) {
            int c = s.charAt(l) - 'a';
            if (sCount[c] == pCount[c]) match--;
            sCount[c]--;
            l++;
            if (sCount[c] == pCount[c]) match++;

            c = s.charAt(r) - 'a';
            if (sCount[c] == pCount[c]) match--;
            sCount[c]++;
            if (sCount[c] == pCount[c]) match++;

            if (match == 26) res.add(l);
        }

        return res;
    }
}

class Solution {
public:
    vector<int> findAnagrams(string s, string p) {
        int n = s.size(), m = p.size();
        if (m > n) return {};

        vector<int> pCount(26, 0), sCount(26, 0);
        for (int i = 0; i < m; i++) {
            pCount[p[i] - 'a']++;
            sCount[s[i] - 'a']++;
        }

        int match = 0;
        for (int i = 0; i < 26; i++) {
            if (pCount[i] == sCount[i]) match++;
        }

        vector<int> res;
        if (match == 26) res.push_back(0);

        int l = 0;
        for (int r = m; r < n; r++) {
            int c = s[l] - 'a';
            if (sCount[c] == pCount[c]) match--;
            sCount[c]--;
            l++;
            if (sCount[c] == pCount[c]) match++;

            c = s[r] - 'a';
            if (sCount[c] == pCount[c]) match--;
            sCount[c]++;
            if (sCount[c] == pCount[c]) match++;

            if (match == 26) res.push_back(l);
        }

        return res;
    }
};

class Solution {
    /**
     * @param {string} s
     * @param {string} p
     * @return {number[]}
     */
    findAnagrams(s, p) {
        const n = s.length,
            m = p.length;
        if (m > n) return [];

        const pCount = new Array(26).fill(0);
        const sCount = new Array(26).fill(0);

        for (let i = 0; i < m; i++) {
            pCount[p.charCodeAt(i) - 97]++;
            sCount[s.charCodeAt(i) - 97]++;
        }

        let match = 0;
        for (let i = 0; i < 26; i++) {
            if (pCount[i] === sCount[i]) match++;
        }

        const res = [];
        if (match === 26) res.push(0);

        let l = 0;
        for (let r = m; r < n; r++) {
            let c = s.charCodeAt(l) - 97;
            if (sCount[c] === pCount[c]) match--;
            sCount[c]--;
            l++;
            if (sCount[c] === pCount[c]) match++;

            c = s.charCodeAt(r) - 97;
            if (sCount[c] === pCount[c]) match--;
            sCount[c]++;
            if (sCount[c] === pCount[c]) match++;

            if (match === 26) res.push(l);
        }

        return res;
    }
}

public class Solution {
    public IList<int> FindAnagrams(string s, string p) {
        int n = s.Length, m = p.Length;
        if (m > n) return new List<int>();

        int[] pCount = new int[26], sCount = new int[26];
        for (int i = 0; i < m; i++) {
            pCount[p[i] - 'a']++;
            sCount[s[i] - 'a']++;
        }

        int match = 0;
        for (int i = 0; i < 26; i++) {
            if (pCount[i] == sCount[i]) match++;
        }

        List<int> res = new List<int>();
        if (match == 26) res.Add(0);

        int l = 0;
        for (int r = m; r < n; r++) {
            int c = s[l] - 'a';
            if (sCount[c] == pCount[c]) match--;
            sCount[c]--;
            l++;
            if (sCount[c] == pCount[c]) match++;

            c = s[r] - 'a';
            if (sCount[c] == pCount[c]) match--;
            sCount[c]++;
            if (sCount[c] == pCount[c]) match++;

            if (match == 26) res.Add(l);
        }

        return res;
    }
}

class Solution {
    fun findAnagrams(s: String, p: String): List<Int> {
        val n = s.length
        val m = p.length
        if (m > n) return emptyList()

        val pCount = IntArray(26)
        val sCount = IntArray(26)
        for (i in 0 until m) {
            pCount[p[i] - 'a']++
            sCount[s[i] - 'a']++
        }

        var match = 0
        for (i in 0 until 26) {
            if (pCount[i] == sCount[i]) match++
        }

        val res = mutableListOf<Int>()
        if (match == 26) res.add(0)

        var l = 0
        for (r in m until n) {
            var c = s[l] - 'a'
            if (sCount[c] == pCount[c]) match--
            sCount[c]--
            l++
            if (sCount[c] == pCount[c]) match++

            c = s[r] - 'a'
            if (sCount[c] == pCount[c]) match--
            sCount[c]++
            if (sCount[c] == pCount[c]) match++

            if (match == 26) res.add(l)
        }

        return res
    }
}

class Solution {
    func findAnagrams(_ s: String, _ p: String) -> [Int] {
        let n = s.count, m = p.count
        if m > n { return [] }

        var pCount = [Int](repeating: 0, count: 26)
        var sCount = [Int](repeating: 0, count: 26)
        let sArr = Array(s)
        let pArr = Array(p)

        for i in 0..<m {
            pCount[Int(pArr[i].asciiValue!) - 97] += 1
            sCount[Int(sArr[i].asciiValue!) - 97] += 1
        }

        var match = 0
        for i in 0..<26 {
            if pCount[i] == sCount[i] { match += 1 }
        }

        var res = [Int]()
        if match == 26 { res.append(0) }

        var l = 0
        for r in m..<n {
            var c = Int(sArr[l].asciiValue!) - 97
            if sCount[c] == pCount[c] { match -= 1 }
            sCount[c] -= 1
            l += 1
            if sCount[c] == pCount[c] { match += 1 }

            c = Int(sArr[r].asciiValue!) - 97
            if sCount[c] == pCount[c] { match -= 1 }
            sCount[c] += 1
            if sCount[c] == pCount[c] { match += 1 }

            if match == 26 { res.append(l) }
        }

        return res
    }
}

Time & Space Complexity

Time complexity: $O(n + m)$
Space complexity: $O(1)$ since we have at most $26$ different characters.

Where $n$ is the length of the string $s$ and $m$ is the length of the string $p$ .

Common Pitfalls

Forgetting Edge Case When Pattern is Longer Than String

If the pattern p is longer than the string s, there cannot be any anagrams. Failing to handle this edge case at the start can lead to index out of bounds errors or incorrect empty results being returned for the wrong reason.

Incorrect Window Size Management

When sliding the window, it is easy to make off-by-one errors with the window boundaries. The window must always be exactly len(p) characters wide. Adding a character before removing one, or vice versa, can temporarily create windows of incorrect size and lead to false matches.

Inefficient Frequency Comparison

Comparing two frequency arrays or maps after every window slide takes O(26) or O(k) time where k is the alphabet size. While this is technically constant, it adds overhead. The optimal approach tracks how many character counts currently match between the window and pattern, updating this count incrementally as characters enter and leave the window.