Longest Palindromic Substring

来源：Leetcode

原帖：http://oj.leetcode.com/problems/longest-palindromic-substring/

题目：
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.

代码：

 class Solution {  
 public:  
   string longestPalindrome(string s) {  
     int N = s.length();  
     int start = 0, maxlen = 0;  
     for (int center = 0; center < N; center++) {  
       expand(s, center, center, maxlen, start);  
       expand(s, center, center + 1, maxlen, start);  
     }  
     return s.substr(start, maxlen);  
   }  
     
   void expand(const string& s, int left, int right, int& maxlen, int& start){  
     while (left >= 0 && right < s.length() && s[left] == s[right]) {  
       left--;   
       right++;  
     }  
     if (right - left - 1 > maxlen) {  
       maxlen = right - left - 1;  
       start = left + 1;  
     }  
   }  
 };