Leetcode-Longest Palindromic Substring

Given a string S, find the longest palindromic substring in S. You may assume that the maximum length of S is 1000, and there exists one unique longest palindromic substring.

Analysis:

We define the state d[i][j] as whether S[j-i+1...j] is palindromic, i.e., i is length, j the end of the substring.

d[i][j] is true, only when the char S[j-i+1]==S[j] and S[j-i+1...j-1] is palindromic, i.e., d[i-2][j-1] is true.

NOTE: the function substring() is time consuming.

Solution:

public class Solution {
    public String longestPalindrome(String s) {
        if (s.length()==0 || s.length()==1) return s;
        String res = "";
        boolean[][] valid = new boolean[s.length()+1][s.length()];
        Arrays.fill(valid[0],true);
        Arrays.fill(valid[1],true);
        int start = 0, end = 0;
        for (int i=2;i<=s.length();i++)
            for (int j=i-1;j<s.length();j++)
                if (valid[i-2][j-1] && s.charAt(j-i+1)==s.charAt(j)){
                    valid[i][j]=true;
                    //res = s.substring(j-i+1,j+1);
                    start = j-i+1;
                    end = j+1;
                } else valid[i][j]=false;

        return s.substring(start,end);        
    }
}