[leetcode] Longest Palindromic Substring *

/*
 * DP
 *
 * d[i][j] represents the length of longest Palindromic
 *  substring of s[i...j].
 *
 *            |- j-i+1  (s[i] == s[j] && s[i+1...j-1] is Palindrome)
 *  d[i][j] = |
 *            |- max{ d[i+1][j], d[i][j-1] }
 *
 *  Note: use circular array.
 */

#include <string>
#include <iostream>
using namespace std;

class Solution {
public:
    string longestPalindrome(string s) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        int len = s.length();

        int d[3][1000];
        int st[3][1000];
        for(int i=0; i<len; i++){
            d[1][i] = 1;
            st[1][i] = i;
        }

        int idx2 = 2, idx1 = 1, idx0 = 0;
        // DP
        for(int l=2; l<=len; l++){
            for(int i=0; i<=len-l; i++){
                if(s[i] == s[i+l-1] && (l == 2 || d[idx0][i+1] == l-2)){
                    d[idx2][i] = l;
                    st[idx2][i] = i;
                }
                else if(d[idx1][i+1] > d[idx1][i]){
                    d[idx2][i] = d[idx1][i+1];
                    st[idx2][i] = st[idx1][i+1];
                }
                else{
                    d[idx2][i] = d[idx1][i];
                    st[idx2][i] = st[idx1][i];
                }
            }
            idx2 = (idx2 + 1) % 3;
            idx1 = (idx1 + 1) % 3;
            idx0 = (idx0 + 1) % 3;

        }

        s[st[idx1][0]+d[idx1][0]] = 0;
        return &s[st[idx1][0]];
    }
};


int main(){
    string s = "abccbd";

    cout << (new Solution())->longestPalindrome(s) << endl;

    return 0;
}