Q29 Longest Palindromic Subsequence - LeetCode 516

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Sun Mar 26 2023 07:52:04 GMT+0000 (Coordinated Universal Time)

Saved by @Ayush_dabas07

class Solution {
    public int longestPalindromeSubseq(String s) {
        /* Q29 of dp series , gap1 strategy 
        */

        //making 2d dp array, S&m - every cell denotes LPS of that string
        int n = s.length();
        int[][] dp = new int[n][n];

        for(int gap = 0 ; gap < n ; gap++){
                
            for(int i = 0 , j = gap; j < n ; i++ , j++){
                if(gap == 0)
                dp[i][j] = 1;   //for 1 character string LPS = 1

                //for 2char string , LPS = 2 if c1=c2 else LPS = 1
                else if(gap == 1)
                dp[i][j] = s.charAt(i)==s.charAt(j) ? 2 : 1;
                

                //i denotes prefix , j denotes suffix
                else{
                    
                    //if c1 == c2 , LCS of middle part + 2 for both chars
                    if(s.charAt(i) == s.charAt(j))
                    dp[i][j] = dp[i+1][j-1] + 2;

                    //else max of (prefix , suffix)
                    else
                    dp[i][j] = Math.max(dp[i][j-1] , dp[i+1][j]);
                }
            }
        }

        // for(int i = 0 ; i < n ; i++){
        //     for(int j =  0 ; j < n ;j++)
        //     System.out.print(dp[i][j] + " ");

        //     System.out.println();
        // }
        

        return dp[0][n-1];
    }
}

https://leetcode.com/problems/longest-palindromic-subsequence/

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