516. Longest Palindromic Subsequence

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題目敘述

Given a string s, find the longest palindromic subsequence’s length in s.

A subsequence is a sequence that can be derived from another sequence by deleting some or no elements without changing the order of the remaining elements.

Example 1:

Input: s = “bbbab” Output: 4 Explanation: One possible longest palindromic subsequence is “bbbb”.

Example 2:

Input: s = “cbbd” Output: 2 Explanation: One possible longest palindromic subsequence is “bb”.

解題思路

Solution

class Solution {
    public int longestPalindromeSubseq(String s) {
        int n = s.length();
        int[] dp = new int[n];

        for (int i = n - 1; i >= 0; i--) {
            int[] newdp = new int[n];
            newdp[i] = 1;
            for (int j = i + 1; j < n; j++) {
                if (s.charAt(i) == s.charAt(j)) {
                    newdp[j] = 2 + dp[j-1];
                } else {
                    newdp[j] = Math.max(dp[j], newdp[j-1]);
                }
            }
            dp = newdp;
        }
        
        return dp[n-1];
    }
}