Submission #19424

#	Time	Username	Problem	Language	Result	Execution time	Memory
19424		Qwaz	Ω (kriii4_P3)	C++14	100 / 100	0 ms	1084 KiB

P3

#include <cstdio>

typedef long long ll;
const int MOD = 1000000007, MAX = 110;;

ll modpow(ll a, ll x) {
	ll ret = 1;
	a = a % MOD;

	while (x) {
		if (x & 1) ret = ret * a % MOD;
		a = a * a % MOD;
		x >>= 1;
	}

	return ret;
}

int p, q, n, k;

ll coeff[MAX], constant[MAX], val[MAX];

int main() {
	scanf("%d%d%d%d", &p, &q, &n, &k);

	if (k == 0) puts("0");
	else if (k == n) puts("1");
	else {
		ll x = q * modpow(p, MOD-2) % MOD;
		ll y = 1 - x;
		if (y < 0) y += MOD;

		coeff[n] = 0;
		constant[n] = 1;
		for (int i = n-1; i > 0; i--) {
			ll div = 1 - y * coeff[i+1];
			div %= MOD;
			if (div < 0) div += MOD;
			div = modpow(div, MOD-2);

			coeff[i] = x * div % MOD;
			constant[i] = (y * constant[i+1] % MOD) * div % MOD;
		}

		val[0] = 0;
		val[1] = constant[1];
		for (int i = 2; i < n; i++) {
			val[i] = (coeff[i] * val[i-1] + constant[i]) % MOD;
		}

		printf("%lld\n", val[k]);
	}

	return 0;
}

• Subtask 1: 100 / 100