680. Valid Palindrome II - Explanation

Description

You are given a string s, return true if the s can be a palindrome after deleting at most one character from it.

A palindrome is a string that reads the same forward and backward.

Note: Alphanumeric characters consist of letters (A-Z, a-z) and numbers (0-9).

Example 1:

Input: s = "aca"

Output: true

Explanation: "aca" is already a palindrome.

Example 2:

Input: s = "abbadc"

Output: false

Explanation: "abbadc" is not a palindrome and can't be made a palindrome after deleting at most one character.

Example 3:

Input: s = "abbda"

Output: true

Explanation: "We can delete the character 'd'.

Constraints:

1 <= s.length <= 100,000
s is made up of only lowercase English letters.

Topics

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Prerequisites

Before attempting this problem, you should be comfortable with:

Two Pointers Technique - Using pointers from both ends of a string to compare characters
Palindrome Fundamentals - Understanding how to check if a string reads the same forwards and backwards

1. Brute Force

Intuition

The simplest approach is to check every possibility. First, check if the string is already a palindrome. If not, try removing each character one at a time and check if the resulting string becomes a palindrome. If any removal produces a palindrome, return true. This guarantees we find a solution if one exists, but it requires checking up to n different strings.

Algorithm

First check if the original string is a palindrome by comparing it to its reverse. If yes, return true.
For each index i from 0 to n-1:
- Create a new string by removing the character at index i.
- Check if this new string is a palindrome.
- If it is, return true.
If no single removal produces a palindrome, return false.

class Solution:
    def validPalindrome(self, s: str) -> bool:
        if s == s[::-1]:
            return True

        for i in range(len(s)):
            newS = s[:i] + s[i + 1:]
            if newS == newS[::-1]:
                return True

        return False

public class Solution {
    public boolean validPalindrome(String s) {
        if (isPalindrome(s)) {
            return true;
        }

        for (int i = 0; i < s.length(); i++) {
            String newS = s.substring(0, i) + s.substring(i + 1);
            if (isPalindrome(newS)) {
                return true;
            }
        }

        return false;
    }

    private boolean isPalindrome(String s) {
        int left = 0, right = s.length() - 1;
        while (left < right) {
            if (s.charAt(left) != s.charAt(right)) {
                return false;
            }
            left++;
            right--;
        }
        return true;
    }
}

class Solution {
public:
    bool validPalindrome(string s) {
        if (isPalindrome(s)) {
            return true;
        }

        for (int i = 0; i < s.size(); i++) {
            string newS = s.substr(0, i) + s.substr(i + 1);
            if (isPalindrome(newS)) {
                return true;
            }
        }

        return false;
    }

private:
    bool isPalindrome(const string& s) {
        int left = 0, right = s.size() - 1;
        while (left < right) {
            if (s[left] != s[right]) {
                return false;
            }
            left++;
            right--;
        }
        return true;
    }
};

class Solution {
    /**
     * @param {string} s
     * @return {boolean}
     */
    validPalindrome(s) {
        if (this.isPalindrome(s)) {
            return true;
        }

        for (let i = 0; i < s.length; i++) {
            const newS = s.slice(0, i) + s.slice(i + 1);
            if (this.isPalindrome(newS)) {
                return true;
            }
        }

        return false;
    }

    /**
     * @param {string} s
     * @return {boolean}
     */
    isPalindrome(s) {
        let left = 0,
            right = s.length - 1;
        while (left < right) {
            if (s[left] !== s[right]) {
                return false;
            }
            left++;
            right--;
        }
        return true;
    }
}

public class Solution {
    public bool ValidPalindrome(string s) {
        if (IsPalindrome(s)) return true;

        for (int i = 0; i < s.Length; i++) {
            string newS = s.Substring(0, i) + s.Substring(i + 1);
            if (IsPalindrome(newS)) return true;
        }

        return false;
    }

    private bool IsPalindrome(string str) {
        int left = 0, right = str.Length - 1;
        while (left < right) {
            if (str[left] != str[right]) return false;
            left++;
            right--;
        }
        return true;
    }
}

func validPalindrome(s string) bool {
    isPalindrome := func(str string) bool {
        left, right := 0, len(str)-1
        for left < right {
            if str[left] != str[right] {
                return false
            }
            left++
            right--
        }
        return true
    }

    if isPalindrome(s) {
        return true
    }

    for i := 0; i < len(s); i++ {
        newS := s[:i] + s[i+1:]
        if isPalindrome(newS) {
            return true
        }
    }

    return false
}

class Solution {
    fun validPalindrome(s: String): Boolean {
        fun isPalindrome(str: String): Boolean {
            var left = 0
            var right = str.length - 1
            while (left < right) {
                if (str[left] != str[right]) return false
                left++
                right--
            }
            return true
        }

        if (isPalindrome(s)) return true

        for (i in s.indices) {
            val newS = s.substring(0, i) + s.substring(i + 1)
            if (isPalindrome(newS)) return true
        }

        return false
    }
}

class Solution {
    func validPalindrome(_ s: String) -> Bool {
        func isPalindrome(_ str: String) -> Bool {
            let arr = Array(str)
            var left = 0
            var right = arr.count - 1
            while left < right {
                if arr[left] != arr[right] { return false }
                left += 1
                right -= 1
            }
            return true
        }

        if isPalindrome(s) { return true }

        let arr = Array(s)
        for i in 0..<arr.count {
            let newS = String(arr[0..<i]) + String(arr[(i+1)...])
            if isPalindrome(newS) { return true }
        }

        return false
    }
}

Time & Space Complexity

Time complexity: $O(n ^ 2)$
Space complexity: $O(n)$

2. Two Pointers

Intuition

Instead of blindly trying every removal, we can be smarter. Use two pointers starting from both ends of the string and move them inward. As long as characters match, keep going. When we find a mismatch, we know exactly where the problem is. At this point, we have only two choices: remove the left character or remove the right character. We check if either choice results in a palindrome for the remaining substring.

Algorithm

Initialize two pointers: l at the start and r at the end of the string.
While l < r:
- If s[l] == s[r], move both pointers inward (l++, r--).
- If they differ, we found the mismatch. Check two possibilities:
  - Skip the left character and check if s[l+1...r] is a palindrome.
  - Skip the right character and check if s[l...r-1] is a palindrome.
- Return true if either substring is a palindrome.
If the loop completes without mismatches, the string is already a palindrome. Return true.

class Solution:
    def validPalindrome(self, s: str) -> bool:
        l, r = 0, len(s) - 1

        while l < r:
            if s[l] != s[r]:
                skipL = s[l + 1 : r + 1]
                skipR = s[l : r]
                return skipL == skipL[::-1] or skipR == skipR[::-1]
            l, r = l + 1, r - 1

        return True

public class Solution {
    public boolean validPalindrome(String s) {
        int l = 0, r = s.length() - 1;

        while (l < r) {
            if (s.charAt(l) != s.charAt(r)) {
                return isPalindrome(s.substring(0, l) + s.substring(l + 1)) ||
                       isPalindrome(s.substring(0, r) + s.substring(r + 1));
            }
            l++;
            r--;
        }

        return true;
    }

    private boolean isPalindrome(String s) {
        int l = 0, r = s.length() - 1;
        while (l < r) {
            if (s.charAt(l) != s.charAt(r)) {
                return false;
            }
            l++;
            r--;
        }
        return true;
    }
}

class Solution {
public:
    bool validPalindrome(string s) {
        int l = 0, r = s.size() - 1;

        while (l < r) {
            if (s[l] != s[r]) {
                return isPalindrome(s.substr(0, l) + s.substr(l + 1)) ||
                       isPalindrome(s.substr(0, r) + s.substr(r + 1));
            }
            l++;
            r--;
        }

        return true;
    }

private:
    bool isPalindrome(string s) {
        int l = 0, r = s.length() - 1;
        while (l < r) {
            if (s[l] != s[r]) {
                return false;
            }
            l++;
            r--;
        }
        return true;
    }
};

class Solution {
    /**
     * @param {string} s
     * @return {boolean}
     */
    validPalindrome(s) {
        let l = 0,
            r = s.length - 1;

        while (l < r) {
            if (s[l] !== s[r]) {
                return (
                    this.isPalindrome(s.slice(0, l) + s.slice(l + 1)) ||
                    this.isPalindrome(s.slice(0, r) + s.slice(r + 1))
                );
            }
            l++;
            r--;
        }

        return true;
    }

    /**
     * @param {string} s
     * @return {boolean}
     */
    isPalindrome(s) {
        let left = 0,
            right = s.length - 1;
        while (left < right) {
            if (s[left] !== s[right]) {
                return false;
            }
            left++;
            right--;
        }
        return true;
    }
}

public class Solution {
    public bool ValidPalindrome(string s) {
        int l = 0, r = s.Length - 1;

        while (l < r) {
            if (s[l] != s[r]) {
                string skipL = s.Substring(l + 1, r - l);
                string skipR = s.Substring(l, r - l);
                return IsPalindrome(skipL) || IsPalindrome(skipR);
            }
            l++;
            r--;
        }

        return true;
    }

    private bool IsPalindrome(string str) {
        int left = 0, right = str.Length - 1;
        while (left < right) {
            if (str[left] != str[right]) return false;
            left++;
            right--;
        }
        return true;
    }
}

func validPalindrome(s string) bool {
    isPalindrome := func(str string) bool {
        left, right := 0, len(str)-1
        for left < right {
            if str[left] != str[right] {
                return false
            }
            left++
            right--
        }
        return true
    }

    l, r := 0, len(s)-1
    for l < r {
        if s[l] != s[r] {
            skipL := s[l+1 : r+1]
            skipR := s[l:r]
            return isPalindrome(skipL) || isPalindrome(skipR)
        }
        l++
        r--
    }

    return true
}

class Solution {
    fun validPalindrome(s: String): Boolean {
        fun isPalindrome(str: String): Boolean {
            var left = 0
            var right = str.length - 1
            while (left < right) {
                if (str[left] != str[right]) return false
                left++
                right--
            }
            return true
        }

        var l = 0
        var r = s.length - 1

        while (l < r) {
            if (s[l] != s[r]) {
                val skipL = s.substring(l + 1, r + 1)
                val skipR = s.substring(l, r)
                return isPalindrome(skipL) || isPalindrome(skipR)
            }
            l++
            r--
        }

        return true
    }
}

class Solution {
    func validPalindrome(_ s: String) -> Bool {
        func isPalindrome(_ str: String) -> Bool {
            let arr = Array(str)
            var left = 0
            var right = arr.count - 1
            while left < right {
                if arr[left] != arr[right] { return false }
                left += 1
                right -= 1
            }
            return true
        }

        let arr = Array(s)
        var l = 0
        var r = arr.count - 1

        while l < r {
            if arr[l] != arr[r] {
                let skipL = String(arr[(l+1)...(r)])
                let skipR = String(arr[l..<r])
                return isPalindrome(skipL) || isPalindrome(skipR)
            }
            l += 1
            r -= 1
        }

        return true
    }
}

Time & Space Complexity

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

3. Two Pointers (Optimal)

Intuition

The previous two-pointer solution creates new substrings, which costs O(n) space. We can optimize this by passing index bounds to our palindrome check function instead of creating new strings. This way, we check the same characters without allocating extra memory. The logic remains identical: find the first mismatch, then verify if skipping either character leads to a valid palindrome.

Algorithm

Create a helper function isPalindrome(l, r) that checks if s[l...r] is a palindrome using two pointers without creating a new string.
Initialize pointers l = 0 and r = len(s) - 1.
While l < r:
- If s[l] != s[r], return isPalindrome(l+1, r) or isPalindrome(l, r-1).
- Otherwise, increment l and decrement r.
Return true if the loop completes (string is already a palindrome).

class Solution:
    def validPalindrome(self, s: str) -> bool:
        def is_palindrome(l, r):
            while l < r:
                if s[l] != s[r]:
                    return False
                l += 1
                r -= 1
            return True

        l, r = 0, len(s) - 1
        while l < r:
            if s[l] != s[r]:
                return (is_palindrome(l + 1, r) or
                        is_palindrome(l, r - 1))
            l += 1
            r -= 1

        return True

public class Solution {
    public boolean validPalindrome(String s) {
        int l = 0, r = s.length() - 1;

        while (l < r) {
            if (s.charAt(l) != s.charAt(r)) {
                return isPalindrome(s, l + 1, r) ||
                       isPalindrome(s, l, r - 1);
            }
            l++;
            r--;
        }

        return true;
    }

    private boolean isPalindrome(String s, int l, int r) {
        while (l < r) {
            if (s.charAt(l) != s.charAt(r)) {
                return false;
            }
            l++;
            r--;
        }
        return true;
    }
}

class Solution {
public:
    bool validPalindrome(string s) {
        int l = 0, r = s.size() - 1;

        while (l < r) {
            if (s[l] != s[r]) {
                return isPalindrome(s, l + 1, r) ||
                       isPalindrome(s, l, r - 1);
            }
            l++;
            r--;
        }

        return true;
    }

private:
    bool isPalindrome(const string& s, int l, int r) {
        while (l < r) {
            if (s[l] != s[r]) {
                return false;
            }
            l++;
            r--;
        }
        return true;
    }
};

class Solution {
    /**
     * @param {string} s
     * @return {boolean}
     */
    validPalindrome(s) {
        let l = 0,
            r = s.length - 1;

        while (l < r) {
            if (s[l] !== s[r]) {
                return (
                    this.isPalindrome(s, l + 1, r) ||
                    this.isPalindrome(s, l, r - 1)
                );
            }
            l++;
            r--;
        }

        return true;
    }

    /**
     * @param {string} s
     * @param {number} l
     * @param {number} r
     * @return {boolean}
     */
    isPalindrome(s, l, r) {
        while (l < r) {
            if (s[l] !== s[r]) {
                return false;
            }
            l++;
            r--;
        }
        return true;
    }
}

public class Solution {
    public bool ValidPalindrome(string s) {
        bool IsPalindrome(int l, int r) {
            while (l < r) {
                if (s[l] != s[r]) return false;
                l++;
                r--;
            }
            return true;
        }

        int left = 0, right = s.Length - 1;
        while (left < right) {
            if (s[left] != s[right]) {
                return IsPalindrome(left + 1, right) || IsPalindrome(left, right - 1);
            }
            left++;
            right--;
        }

        return true;
    }
}

func validPalindrome(s string) bool {
    isPalindrome := func(l, r int) bool {
        for l < r {
            if s[l] != s[r] {
                return false
            }
            l++
            r--
        }
        return true
    }

    l, r := 0, len(s)-1
    for l < r {
        if s[l] != s[r] {
            return isPalindrome(l+1, r) || isPalindrome(l, r-1)
        }
        l++
        r--
    }

    return true
}

class Solution {
    fun validPalindrome(s: String): Boolean {
        fun isPalindrome(l: Int, r: Int): Boolean {
            var left = l
            var right = r
            while (left < right) {
                if (s[left] != s[right]) return false
                left++
                right--
            }
            return true
        }

        var l = 0
        var r = s.length - 1
        while (l < r) {
            if (s[l] != s[r]) {
                return isPalindrome(l + 1, r) || isPalindrome(l, r - 1)
            }
            l++
            r--
        }

        return true
    }
}

class Solution {
    func validPalindrome(_ s: String) -> Bool {
        let arr = Array(s)

        func isPalindrome(_ l: Int, _ r: Int) -> Bool {
            var left = l
            var right = r
            while left < right {
                if arr[left] != arr[right] { return false }
                left += 1
                right -= 1
            }
            return true
        }

        var l = 0
        var r = arr.count - 1
        while l < r {
            if arr[l] != arr[r] {
                return isPalindrome(l + 1, r) || isPalindrome(l, r - 1)
            }
            l += 1
            r -= 1
        }

        return true
    }
}

Time & Space Complexity

Time complexity: $O(n)$
Space complexity: $O(1)$

Common Pitfalls

Only Trying One Deletion Option

When a mismatch is found at positions l and r, you must check both possibilities: removing the character at l or removing the character at r. A common mistake is only trying one option (like always removing the left character). Both substrings s[l+1...r] and s[l...r-1] must be checked, and the answer is true if either forms a palindrome.

Forgetting That Zero Deletions Is Valid

The problem asks if the string can become a palindrome by deleting at most one character. This includes deleting zero characters. If the original string is already a palindrome, the answer is true. Some solutions only consider the deletion case and forget to check if the string is already valid as-is.