Leetcode 516. Longest Palindromic Subsequence

Given a string s, find the longest palindromic subsequence's length in s. You may assume that the maximum length of s is 1000.

Example 1:
Input:

"bbbab"

Output:

4

One possible longest palindromic subsequence is "bbbb".

Example 2:
Input:

"cbbd"

Output:

2

One possible longest palindromic subsequence is "bb".

Analysis:
The problem is similar to the longest palindromic substring. So we use dp[i][j] means from character i to j, the longest palindromic substring. 

Code (Java):

class Solution {
    public int longestPalindromeSubseq(String s) {
        if (s == null || s.length() == 0) {
            return 0;
        }
        
        int[][] dp = new int[s.length()][s.length()];
        int maxLen = 0;
        for (int i = s.length() - 1; i >= 0; i--) {
            for (int j = i; j < s.length(); j++) {
                if (s.charAt(i) == s.charAt(j)) {
                    if (j - i < 2) {
                        dp[i][j] = j - i + 1;
                    } else {
                        dp[i][j] = dp[i + 1][j - 1] + 2;
                    }
                } else {
                    dp[i][j] = Math.max(dp[i + 1][j], dp[i][j - 1]);
                }
                
                maxLen = Math.max(maxLen, dp[i][j]);
            }
        }
        
        return maxLen;
    }
}