5. Longest Palindromic Substring

 1 class Solution {
 2     public String longestPalindrome(String s) {
 3         int n = s.length();
 4         if (n == 0) return "";
 5         boolean [][]flag = new boolean[n][n];
 6         for (int i = 0; i < n; ++i) {
 7             flag[i][0] = true;
 8         }
 9         
10         for (int len = 2; len <= n; ++len ) {
11             for (int i = 0; i + len - 1 < n; ++i) {
12                 
13                 if (s.charAt(i) != s.charAt(i + len - 1)) continue;
14                 if (len - 3 == -1 || flag[i + 1][len - 3]) {
15                     flag[i][len - 1] = true;
16                 }
17             }
18         }
19         
20         for (int len = n; len >= 0; --len ) {
21             for (int i = 0; i + len - 1 < n; ++i) {
22                 if (flag[i][len - 1]) return s.substring(i, i + len);
23             }
24         }
25         return "";
26     }
27 }