Kortit II

#include <bits/stdc++.h>
#define ln "\n"
using namespace std;
using ll = long long;
int MOD = 1e9+7;
ll d[2020][2020];
ll f[2020];

ll modExp(ll base, ll exp, int mod) {
    ll result = 1;
    while (exp > 0) {
        if (exp % 2 == 1) {
            result = (result * base) % mod;
        }
        base = (base * base) % mod;
        exp /= 2;
    }
    return result;
}
 
 
void construct() {
	int n = 2000; int k = 2000;
	
	for (int i = 2; i <= n; i++) d[i][1] = 1;
 
	for (int i = 3; i <= n; i++) {
		for (int j = 2; j <= k; j++) {
			d[i][j] = (i - 1) * (d[i - 2][j - 1]) + (i - j) * d[i - 1][j - 1] + j * (d[i - 1][j]);
			d[i][j] %= MOD;
		}
	}
 
	ll s = 1;
	for (ll i = 1; i <= k; i++) {
		s *= i;
		s %= MOD;
		f[i] = s;
	}
}


int main() {
    construct();
    int t;
    cin >> t;
    while (t--) {
        int n, a, b;
        cin >> n >> a >> b;
        if ((a == 0 && b != 0) || (a != 0 && b == 0) || (a + b > n)) {
            cout << 0 << ln;
        } else {
            ll variations = d[a+b][a];
            if(variations == 0) variations = 1;
            ll c = a + b;

            ll finalMul = 1;
            if(c != 0) {
                finalMul = (finalMul * f[c]) % MOD;
            }
            for (ll i = c + 1; i <= n; i++) {
                finalMul = (finalMul * i) % MOD;
                finalMul = (finalMul * i) % MOD;
            }

            ll diver = 1;
            diver = (diver * f[n-c]) % MOD;
            if(diver == 0) diver = 1;
            ll diverInv = modExp(diver, MOD - 2, MOD);  
            
            finalMul = (finalMul * variations) % MOD;
            finalMul = (finalMul * diverInv) % MOD;

            cout << finalMul << ln;
        }
    }
}

Test 1

Group: 1, 2, 3, 4, 5

Verdict: ACCEPTED

Test 2

Group: 2, 3, 4, 5

Verdict: ACCEPTED

Test 3

Group: 3, 4, 5

Verdict: ACCEPTED

Test 4

Group: 4, 5

Verdict: ACCEPTED

Test 5

Group: 5

Verdict: ACCEPTED

Test 6

Group: 5

Verdict: ACCEPTED