最长回文子串

Example 1:
Input: s = "babad"
Output: "bab"
Explanation: "aba" is also a valid answer.

Example 2:
Input: s = "cbbd"
Output: "bb"

var longestPalindrome = function(s) {
     let res = "";
    let max = 0;

    for(let i = 0; i < s.length; i++){
        for(let j = 0; j <= 1; j++){
            let l = i;
            let r = i + j;

            while(l >= 0 && r < s.length && s[l] === s[r]){
                let len = r - l + 1;
                if(len > max){
                    res = s.substring(l, r + 1);
                    max = len;
                }
                l--;
                r++;
            }
        }
    }
    return res;
};