length of LCS using recursion

class Solution {
public:
    int LCS(vector<vector<int>> &dp,string s1,string s2,int m,int n)
    {
        if(dp[m][n]!=-1) return dp[m][n];
        if(n==0||m==0) return dp[m][n]=0;
        if(s1[m-1]==s2[n-1])
            return dp[m][n]=1+LCS(dp,s1,s2,m-1,n-1);
        else
            return dp[m][n]=max(LCS(dp,s1,s2,m-1,n),LCS(dp,s1,s2,m,n-1));
    }
    int longestCommonSubsequence(string t1, string t2) {
        int m=t1.length(),n=t2.length();
        vector<vector<int>> dp(m+1,vector<int>(n+1,-1));
        return LCS(dp,t1,t2,m,n);
    }
};