125. Valid Palindrome

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

• T: $O(N)$
• S: $O(1)$