125. Valid Palindrome

Problem Description

A phrase is a palindrome if, after converting all uppercase letters into lowercase letters and removing all non-alphanumeric characters, it reads the same forward and backward. Alphanumeric characters include letters and numbers.

Given a string s, return true if it is a palindrome, or false otherwise.

Examples

Example 1:
Input: s = "A man, a plan, a canal: Panama"
Output: true
Explanation: "amanaplanacanalpanama" is a palindrome.

Example 2:
Input: s = "race a car"
Output: false
Explanation: "raceacar" is not a palindrome.

Example 3:
Input: s = " "
Output: true
Explanation: s is an empty string "" after removing non-alphanumeric characters.
Since an empty string reads the same forward and backward, it is a palindrome.

def isPalindrome(s: str) -> bool:
    # Convert to lowercase and remove non-alphanumeric characters
    filtered_chars = [c.lower() for c in s if c.isalnum()]
    
    # Compare string with its reverse
    return filtered_chars == filtered_chars[::-1]

class Solution {
    public boolean isPalindrome(String s) {
        // Two pointers from both ends
        int left = 0;
        int right = s.length() - 1;
        
        while (left < right) {
            // Skip non-alphanumeric characters from left
            while (left < right && !Character.isLetterOrDigit(s.charAt(left))) {
                left++;
            }
            
            // Skip non-alphanumeric characters from right
            while (left < right && !Character.isLetterOrDigit(s.charAt(right))) {
                right--;
            }
            
            // Compare characters
            if (Character.toLowerCase(s.charAt(left)) != 
                Character.toLowerCase(s.charAt(right))) {
                return false;
            }
            
            left++;
            right--;
        }
        
        return true;
    }
}

class Solution {
public:
    bool isPalindrome(string s) {
        // Two pointers from both ends
        int left = 0;
        int right = s.length() - 1;
        
        while (left < right) {
            // Skip non-alphanumeric characters from left
            while (left < right && !isalnum(s[left])) {
                left++;
            }
            
            // Skip non-alphanumeric characters from right
            while (left < right && !isalnum(s[right])) {
                right--;
            }
            
            // Compare characters
            if (tolower(s[left]) != tolower(s[right])) {
                return false;
            }
            
            left++;
            right--;
        }
        
        return true;
    }
};

/**
 * @param {string} s
 * @return {boolean}
 */
var isPalindrome = function(s) {
    // Convert to lowercase and remove non-alphanumeric characters
    s = s.toLowerCase().replace(/[^a-z0-9]/g, '');
    
    // Two pointers from both ends
    let left = 0;
    let right = s.length - 1;
    
    while (left < right) {
        if (s[left] !== s[right]) {
            return false;
        }
        left++;
        right--;
    }
    
    return true;
};

public class Solution {
    public bool IsPalindrome(string s) {
        // Two pointers from both ends
        int left = 0;
        int right = s.Length - 1;
        
        while (left < right) {
            // Skip non-alphanumeric characters from left
            while (left < right && !Char.IsLetterOrDigit(s[left])) {
                left++;
            }
            
            // Skip non-alphanumeric characters from right
            while (left < right && !Char.IsLetterOrDigit(s[right])) {
                right--;
            }
            
            // Compare characters
            if (Char.ToLower(s[left]) != Char.ToLower(s[right])) {
                return false;
            }
            
            left++;
            right--;
        }
        
        return true;
    }
}