https://leetcode-cn.com/problems/longest-palindromic-substring/
    中心扩散法

    1. class Solution {
    2.     public String longestPalindrome(String s) {
    3.         if(s.length() == 0){
    4.             return null;
    5.         }
    6.        int res = 1;
    7.        int ll =0;
    8.        int rr = 0;
    9.        for(int i = 0; i<s.length(); i++){
    10.            //奇数
    11.            int l = i-1;
    12.            int r = i+1;
    13.            while(l>=0&&r<s.length() &&s.charAt(l) == s.charAt(r)){
    14.                int len = r - l +1;
    15.                if(len > res){
    16.                    res = len;
    17.                    ll = l;
    18.                    rr = r;
    19.                }
    20.                l--;
    21.                r++;
    22.            }
    23.            //偶数
    24.         l = i;
    25.         r = i+1;
    26.         while(l>=0&&r<s.length() &&s.charAt(l) == s.charAt(r)){
    27.                int len = r - l +1;
    28.                if(len > res){
    29.                    res = len;
    30.                    ll = l;
    31.                    rr = r;
    32.                }
    33.                l--;
    34.                r++;
    35.            }
    36.        }
    37.        return s.substring(ll, rr+1);
    38.     }
    39. }