Longest Palindromic Substring

longestPalindromicSubstring.js

/**
 * @param {string} s
 * @return {string}
 */
var longestPalindrome = function(s) {
    let res = "";
    let max = 0;

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

        let start = i;
        let end = i+1;
        while(start >= 0 && end <= s.length - 1 && s[start] == s[end]) {
            if(end - start + 1 > max) {
                max = end - start + 1;
                res = s.substr(start, max);
            }
            start--;
            end++;
        }
    }

    return res;
};

// Run and loop and consider the current element to be the center element
// For odd length
// expand outward on both sides from the center element until one of these conditions fail: 
//        i. left pointer is not out of bounds
//        ii. right pointer is not out of bounds
//        iii. value and left pointer is the same as the value at right pointer
// keep checking if the current length is greater the the max value, if yes then update max value
// For even Length
// Do the same except the pointers start from the center and center + 1;

// Time: O(n^2)
// Space: O(1)