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125. Valid Palindrome

Given a string, determine if it is a palindrome by considering only alphanumeric characters and ignoring cases. Note that an empty string is considered a valid palindrome.

Example 1: Input: "A man, a plan, a canal: Panama" Output: true

Example 2: Input: "race a car" Output: false

Solution Code:

class Solution {
    bool isPalindrome(string s) {
        if (s.empty()) return true;
        int j = s.size() - 1;
        int i = 0;
        while (i < j) {
            if (!isalnum(s[j])) {
                j--;
                continue;
            }
            if (!isalnum(s[i])) {
                i++;
                continue;
            }
            if (tolower(s[i++]) != tolower(s[j--])) {
                return false;
            }
        }
        return true;
    }
}

This solution efficiently checks for palindromes by comparing characters from both ends of the string, ignoring non-alphanumeric characters and case differences. The algorithm runs in O(n) time complexity, where n is the length of the string, making it optimal for this problem.

Note: The comparison is done in a case-insensitive manner to ensure uniformity in validation.