Submission #21295

#	Time	Username	Problem	Language	Result	Execution time	Memory
21295		suzy	순열 (kriii4_T)	C++14	100 / 100	369 ms	8928 KiB

T

#include <stdio.h>

#define MOD 1000000007

typedef long long lli;

lli fac[1000002];
lli pw(lli n, lli x) {
	if(x==0) return 1;
	if(x%2) return n*pw(n,x-1)%MOD;
	else { lli ret=pw(n,x/2); return ret*ret%MOD; }
}

int main() {
	lli n, k;
	fac[0]=1;
	for(lli i=1;i<=1000001;i++)
		fac[i]=fac[i-1]*i%MOD;
	scanf("%lld %lld",&n,&k); k++;
	if(k>n) {
		puts("0"); return 0;
	}
	lli res=0, ncl=fac[n+1]*pw(fac[n-k],MOD-2)%MOD*pw(fac[k+1],MOD-2)%MOD;
	for(lli l=k;l<=n;l++) {
		res+=(n-l+1)*fac[l]%MOD*fac[n-l]%MOD*ncl%MOD;
		res%=MOD;
		ncl=ncl*(n-l)%MOD*pw(l+2,MOD-2)%MOD;
	}
	printf("%lld\n",res);
	return 0;
}

Compilation message (stderr)

T.cpp: In function 'int main()':
T.cpp:19:26: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%lld %lld",&n,&k); k++;
                          ^

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