[] LeetCode longest palindromic substring - Dynamic Programming

[Problems] Given a string s, s to find the longest substring palindromic. You can assume that the maximum length of 1000 s.

Example 1 : 
Input: " Babad " 
Output: " BAB " 
Note: " aba " is a valid answer. 
Example 2 : 
Input: " cbbd " 
Output: " BB "

[Thinking] Two days ago we explained the "Center Expansion Act" to solve This question, today we use dynamic programming approach to written question head, we need to look for a recursive formula is as follows:

We will F [i] [j] is expressed as a string of sub-palindromic sequence from j to i, , j <= i, this case is the lower left triangular matrix dp!
If a [i] == a [j ] and f [i-1] [j + 1] = true, then f [i] [j] is also true.

Note that: when ij <2, if s [i] = s [j], then f [i] [j] will be for true, i.e., a single character or two identical adjacent character substring palindromic.

class Solution {
public:
    string longestPalindrome(string s) {
        int slen = s.length();
        if(slen == 0) return "";
        string res = "";
        vector<vector<bool>> f(slen, vector<bool>(slen, false));
        int maxlen = 0;
        int curlen = 0;

        for(int i = 0;i < slen; i++){
            for(int j = 0;j <= i; j++){   // f[0][0]=true, 一定成立
                if((s[i] == s[j]) && ((i-j < 2) || (i > 0 && f[i-1][j+1]))){
                    f[i][j] = true;
                    curlen = i - j + 1;
                    if(curlen > maxlen){
                        maxlen = curlen;
                        res = s.substr(j, curlen);
                    }
                }
            }
        }
        return res;
    }
};