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".

Similar Questions: Longest Palindromic Substring Palindromic Substrings 

Next challenges: Coin Change Integer Break Target Sum

 方法一：迭代。

思路：dp[i][j]表示的字符串i位置到j位置之间（包括i，j）含有的palindromic subsequence的数目，那么有如果s[i]==s[j]，那么dp[i][j]=dp[i+1][j-1]+2；否则dp[i][j]=max(dp[i+1][j],dp[i][j-1])。具体实现的时候，应该先确定i和j位置之间（不包含i和j）的所有短字符串中的palindromic subsequence的数目，然后再求dp[i][j]。

代码：

public class Solution {
    public int longestPalindromeSubseq(String s) {
        int[][] dp = new int[s.length()][s.length()];
        for(int j = 0; j < s.length(); j++) {
            dp[j][j] = 1;
            for(int i = j - 1; i >= 0; i--) {
                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][s.length() -1];
    }
}

方法二：递归。

思路和方法一相同。

代码：

public class Solution {
    public int longestPalindromeSubseq(String s) {
        return helper(s, 0, s.length() - 1, new Integer[s.length()][s.length()]);
    }
    
    private int helper(String s, int i, int j, Integer[][] memo) {
        if(memo[i][j] != null) return memo[i][j];//初始化Integer数组，所以数组中的每个元素是个类，如果类没有初始化就是null
        if(i > j) return 0;
        if(i == j) return 1;
        if(s.charAt(i) == s.charAt(j)) memo[i][j] = helper(s, i + 1, j - 1, memo) + 2;
        else memo[i][j] = Math.max(helper(s, i, j - 1, memo), helper(s, i + 1, j, memo));
        return memo[i][j];
    }
}