[DP] LeetCode 5 Longest Palindromic Substring

$O(N^2)$ code

class Solution {
public:
    string longestPalindrome(string str) {
        int size = str.length();
        int ps = 0, maxLen = 1;
        vector<vector<bool>> isP(size, vector<bool>(size, false));

        // one character string and two character sub-strings
        for (int s = 0; s < size; ++s) {
            isP[s][s] = true;
            int e = s + 1;
            if ((e < size) && (str[s] == str[e])) {
                isP[s][s+1] = true;
                ps =s ; maxLen = 2;
            }
        }

        for (int step = 2; step < size; ++step) {
            for (int s = 0; s + step < size; ++s) {
                int e = s + step;
                if ((str[s] == str[e]) && (isP[s+1][e-1])) {
                isP[s][e] = true;
                ps = s; maxLen = step + 1;
                }
            }
        }

        return str.substr(ps, maxLen);
    }
};