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".

这是一道典型的dp题，用dp[][]来存储i,j两个pointer指向的string的一个char, 分别按照相等和不相等来处理即可。

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