Leetcode5最长回文子串(中心拓展法和动态规划法)

/**
 \* @param {string} s
 \* @return {string}
 */
var longestPalindrome = function longestPalindrome(s){
  let n = s.length;
  let res = '';
  let dp = Array.from(new Array(n),() => new Array(n).fill(false));
​
  for(let i = n-1; i >= 0; i--){
•    for(let j = i; j < n; j++){
•      dp[i][j] = s[i] == s[j] && (j - i < 3 || dp[i+1][j-1]);
     
•      if(dp[i][j] && j - i + 1 > res.length){
•        res = s.substring(i,j+1);
•      }
•    }
  }
  return res;
};

/**
 * @param {string} s
 * @return {string}
 */
var longestPalindrome = function(s) {
    if(s.length < 2) {
        return s
    }
​
    let start = 0
    let maxLength = 1
    function expandAroundCenter(left, right) {
        while(left >= 0 && right < s.length && s[left] === s[right]) {
            if(right - left + 1 > maxLength) {
                maxLength = right - left + 1
                start = left
            }
            left--
            right++
        }
    }
    for(let i=0; i<s.length; i++) {
        expandAroundCenter(i - 1, i + 1)
        expandAroundCenter(i, i + 1)
    }
    return s.substring(start, start + maxLength)
};