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.

1 class Solution { 2 public: 3     string longestPalindrome(string s) { 4         int n = s.size(); 5         if (n <= 1) return s; 6          7         int start = 0; 8         int maxlen = 0; 9         bool ispal[1000][1000] = {
   false};10         11         for (int i = 0; i < n; i++){12             for (int j = i; j >= 0; j--){13                 if (s[i] == s[j] && (ispal[j+1][i-1] || j+1 > i-1)){14                     ispal[j][i] = true;15                     if (i - j + 1 > maxlen){16                         maxlen = i - j + 1;17                         start = j;18                     }19                 }20             }21         }22         23         return s.substr(start, maxlen);24     }25 };