Given a string s, find the longest palindromic substring in s. You may assume that the maximum length of s is 1000.

Example 1:

Input: "babad"
Output: "bab"
Note: "aba" is also a valid answer.

Example 2:

Input: "cbbd"
Output: "bb"

注意边界条件，如没有回文串。

class Solution {
    int dp[1002][1002];
public:
    string longestPalindrome(string str) {
       int n =str.size(); 
       for(int i=0;i<n;i++)
            dp[i][i] = 1;
       int s=0, l = 0;
       for(int j=1;j<n;j++)
           for(int i=0;i<j;i++) {
               if(str[i]==str[j]) {
                   if(j==i+1) dp[i][j]=1;
                   else
                       dp[i][j]=dp[i+1][j-1];
                   if(dp[i][j] && (j-i+1) >= l) {
                       l = j-i+1;
                       s = i;
                   }
               }
           }
        if(l==0) return str.substr(0, 1);
        return str.substr(s, l);
    }
};