Valid Palindrome

Problem Statement

Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases. A palindrome reads the same forward and backward. This is an easy-level problem solved using the two-pointer technique.

Example

Input: s = "A man, a plan, a canal: Panama"

Output: true (ignoring non-alphanumeric characters and case, it reads "amanaplanacanalpanama")

Input: s = "race a car"

Output: false

Code

public class Solution {
    public boolean isPalindrome(String s) {
        int left = 0, right = s.length() - 1;
        
        while (left < right) {
            while (left < right && !Character.isLetterOrDigit(s.charAt(left))) {
                left++;
            }
            while (left < right && !Character.isLetterOrDigit(s.charAt(right))) {
                right--;
            }
            if (Character.toLowerCase(s.charAt(left)) != Character.toLowerCase(s.charAt(right))) {
                return false;
            }
            left++;
            right--;
        }
        return true;
    }
}
            

def is_palindrome(s):
    left, right = 0, len(s) - 1
    
    while left < right:
        while left < right and not s[left].isalnum():
            left += 1
        while left < right and not s[right].isalnum():
            right -= 1
        if s[left].lower() != s[right].lower():
            return False
        left += 1
        right -= 1
    return True

# Example usage
print(is_palindrome("A man, a plan, a canal: Panama"))  # True
print(is_palindrome("race a car"))  # False
            

function isPalindrome(s) {
    let left = 0, right = s.length - 1;
    
    while (left < right) {
        while (left < right && !/[a-zA-Z0-9]/.test(s[left])) {
            left++;
        }
        while (left < right && !/[a-zA-Z0-9]/.test(s[right])) {
            right--;
        }
        if (s[left].toLowerCase() !== s[right].toLowerCase()) {
            return false;
        }
        left++;
        right--;
    }
    return true;
}

// Example usage
console.log(isPalindrome("A man, a plan, a canal: Panama")); // true
console.log(isPalindrome("race a car")); // false
            

Explanation

• Use two pointers: one starting from the left, one from the right.
• Skip non-alphanumeric characters by incrementing or decrementing pointers.
• Compare the characters (case-insensitive) at both pointers.
• If they don’t match, it’s not a palindrome; otherwise, continue until pointers meet.

Note