0005-longest-palindromic-substring.js

/**
 * 5. Longest Palindromic Substring
 * https://leetcode.com/problems/longest-palindromic-substring/
 * Difficulty: Medium
 *
 * Given a string `s`, return the longest palindromic substring in `s`.
 */

/**
 * @param {string} s
 * @return {string}
 */
var longestPalindrome = function(s) {
  let result = '';

  for (let i = 0; i < s.length; i++) {
    const palindrome1 = getExtendedPalindrome(s, i, i);
    const palindrome2 = getExtendedPalindrome(s, i, i + 1);
    const longerPalindrome = palindrome1.length > palindrome2.length
      ? palindrome1
      : palindrome2;

    if (longerPalindrome.length > result.length) {
      result = longerPalindrome;
    }
  }

  return result;
};

function getExtendedPalindrome(s, start, end) {
  while (start >= 0 && end < s.length && s[start] === s[end]) {
    start--;
    end++;
  }

  return s.slice(start + 1, end);
}