125. Valid Palindrome

Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.

Note: For the purpose of this problem, we define empty string as valid palindrome.

Example 1:

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

Example 2:

Input: "race a car"
Output: false

bool isPalindrome(string s) { // time: O(n); space: O(1)
    for (int l = 0, r = s.length() - 1; l < r; ++l, --r) {
        while (l < r && !isalnum(s[l])) ++l;
        while (l < r && !isalnum(s[r])) --r;
        if (tolower(s[l]) != tolower(s[r])) return false;
    }
    return true;
}