Submission #19916

#	Time	Username	Problem	Language	Result	Execution time	Memory
19916		xdoju	순열 (kriii4_T)	C++14	100 / 100	845 ms	1084 KiB

T

#include <stdio.h>

const int MOD = 1000000007;

int mul(int x, int y){ return (long long)x * y % MOD; }

int power(int x, int y){
  if(y == 0) return 1;

  int r = power(x, y / 2);
  r = mul(r, r);
  if(y % 2 == 1) r = mul(r, x);
  return r;
}

int modinv(int x){
  return power(x, MOD - 2);
}

int main(){
  int N, K; scanf("%d%d", &N, &K);

  int v = 1;
  for(int i = 1; i <= N + 1; i++) v = mul(v, i); // fac(N + 1)

  v = mul(v, N + 1);

  int ans = 0;

  for(int k = 1; k <= N; k++){
    v = mul(v, k);
    v = mul(v, modinv(k + 1));
    v = mul(v, N - k + 1);
    v = mul(v, modinv(N - k + 2));

    if(k >= K + 1) ans = (ans + v) % MOD;
  }

  printf("%d\n", ans);
  return 0;
}

• Subtask 1: 8 / 8
• Subtask 2: 92 / 92