5. Longest Palindromic Substring

Input: "babad"

Output: "bab"

Note: "aba" is also a valid answer.

class Solution {
public:
    string longestPalindrome(string s) {
        int len = s.length();
        vector< vector<int> > dp(len, vector<int>(len));
        string res;
        for( int i=0; i<len; ++i ){
            for( int j=i; j>=0; --j ){
                dp[i][j] = ((s[i]==s[j]) && (i-j<2 || dp[i-1][j+1]));
                if( dp[i][j] && (i-j+1)>res.length() )
                    res = s.substr(j, i-j+1);;
            }
        }
        return res;
    }
};