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Longest Palindrome Subsequence

Problem Statement​

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.

Leetcode link

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".

Constraints:​

  • 1 <= s.length <= 1000
  • s consists only of lowercase English letters.

Code​

Python Code
class Solution(object):
    def longestPalindromeSubseq(self, s):

        n = len(s)
        dp = [[0 for j in range(n)] for i in range(n)]

        for i in range(0, n):## that is a starting point
            dp[i][i] = 1
            for j in range(i-1, -1, -1):##this is a terminal point
                if s[i] == s[j]:
                    dp[i][j] = dp[i-1][j+1] + 2
                else:
                    dp[i][j] = max(dp[i-1][j], dp[i][j+1])

        return dp[n-1][0]