920. Number of Music Playlists

class Solution {
public:
   int nn;
   long long mod = 1e9+7;
   long long dp[101][101];
   long long help(int num,int g,int k){
        if(g==0){
            if(num==nn)return 1;
            else return 0;
        }
        if(dp[num][g]!=-1)return dp[num][g];
        long long ans=0;
        ans = (ans + ((nn-num)*help(num+1,g-1,k))%mod)%mod;
        if(num>=k){
            ans = (ans + ((num-k)*help(num,g-1,k))%mod)%mod;
        }
        return dp[num][g] = ans%mod;
    }
    int numMusicPlaylists(int n, int g, int k) {
        nn=n;
        memset(dp,-1,sizeof(dp));
        return help(0,g,k);
    }
};