Why I'm getting TLE even if I'm using memoization?

Question

Here is the question statement

Given two strings s1 and s2, return the length of their longest common subsequence.

A subsequence of a string is a new string generated from the original string with some characters(can be none) deleted without changing the relative order of the remaining characters. (eg, "ace" is a subsequence of "abcde" while "aec" is not). A common subsequence of two strings is a subsequence that is common to both strings.

If there is no common subsequence, return 0.

Here is my code

class Solution {
public:
    int dp[1001][1001];
    int lcs(string s1,string s2,int n,int m){
        if (n==0 || m == 0)
            return 0;
        if(dp[n][m]!=-1)
            return dp[n][m];
        if (s1[n-1] == s2[m-1])
        {
            return dp[n][m]=  1 + lcs(s1,s2,n-1,m-1);
        }
        else
        {
            return dp[n][m]= max(lcs(s1,s2,n-1,m), lcs(s1,s2,n,m-1));
        }
        return dp[n][m];
    }
    int longestCommonSubsequence(string s1, string s2) {
        memset(dp,-1,1001*1001*sizeof(int));
        return lcs(s1,s2, s1.size(), s2.size());
        
    }
};

Thank You

Answer 1

use call by reference in function definition instead of call by value.

int lcs(string &s1,string &s2,int n,int m);

int longestCommonSubsequence(string &s1, string &s2);