21 Longest Palindromic Subsequence.cpp

#include <bits/stdc++.h>
using namespace std;

int LCS(string X, string Y, int n, int m) {
	int dp[n + 1][m + 1]; // DP - matrix

	// base case of recursion --> for initialization of dp - matrix
	for (int i = 0; i <= n; i++)
		for (int j = 0; j <= m; j++)
			if (i == 0 || j == 0)
				dp[i][j] = 0;

	for (int i = 1; i <= n; i++)
		for (int j = 1; j <= m; j++)
			if (X[i - 1] == Y[j - 1]) // when last character is same
				dp[i][j] = 1 + dp[i - 1][j - 1];
			else // when last character is not same -> pick max
				dp[i][j] = max(dp[i][j - 1], dp[i - 1][j]);

	return dp[n][m];
}

int LPS(string X, int n) {
	string rev_X = X;
	reverse(rev_X.begin(), rev_X.end()); // reverse the string // take reversed string as another string of lcs and apply lcs 
	return LCS(X, rev_X, n, n);
}

signed main() {
	string X, Y; cin >> X;
	int n = X.length();

	cout << LPS(X, n) << endl;
	return 0;
}


leetcode:
class Solution {
public:
    int longestPalindromeSubseq(string s) {
        int n = s.length();
        string t ="";
        for(int i = n ; i>=0; i--){
            t.push_back(s[i]);
        }

        int m  = t.length();
        int d[n+1][m+1];

        for(int i = 0 ;i<=n;i++){
            for(int j=0;j<=m;j++){
                if(i==0 || j==0){
                    d[i][j] = 0;
                }
            }
        }
        for(int i=1;i<=n;i++){
            for(int j=1;j<=m;j++){
                if(s[i-1] == t[j-1]){
                    d[i][j] = d[i-1][j-1] +1;
                }else{
                    d[i][j] = max(d[i-1][j] , d[i][j-1]);
                }
            }
        }
        return d[n][m];

    }
};