5. Longest Palindromic Substring

Given a string S, find the longest palindromic substring in S. You may assume that the maximum length of S is 1000, and there exists one unique longest palindromic substring.

思路1：中心扩展法，枚举中心的文职，再在该位置上扩展，记录并更新得到的最长回文串的长度。注意需要区分回文子串是奇数长度还是偶数长度。

1. class Solution {
   public:
      string longestPalindrome(string s) {
       int len = s.length();
       int max_l = 0;//最长回文子串长度
       int max_s = 0;//最长回文子串开头的位置
       int cur_len;
       int cur_s;
       for (int i = 0; i < len; i++) {//字符串的长度是奇数
           for (int j = 0; i + j<len&&i - j>-1; j++) {
               if (s[i + j] != s[i - j]) {
                   break;
               }
               cur_len = j * 2 + 1;
               cur_s = i - j;
           }
           if (cur_len > max_l) {
               max_s = cur_s;
               max_l = cur_len;
           }
           for (int j = 0; i - j > -1 && i + j + 1 < len; j++) {//字符串为偶数长度
               if (s[i + j + 1] != s[i - j]) {
                   break;
               }
               cur_len = 2 * j + 2;
               cur_s = i - j;
           }
           if (cur_len > max_l) {
               max_s = cur_s;
               max_l = cur_len;
           }
       }
       return s.substr(max_s, max_l);
   }
   };