Submission #19820

#	Time	Username	Problem	Language	Result	Execution time	Memory
19820		Namnamseo	순열 (kriii4_T)	C++14	8 / 100	2000 ms	16708 KiB

T

#include<cstdio>
long long mans,c,mul[1000003],fac[1000003],a,b,i,j,s=1,ans,X=1e9+7;
long long pow(long long aa,long long bb){
    if(bb==0) return 1;
    long long ret=pow(aa,bb/2);
    ret=(ret*ret)%X;
    if(bb&1) ret=(ret*aa)%X;
    return ret;
}
long long com(long long aa,long long bb)
{
    return fac[aa]*pow(fac[bb],X-2)%X*pow(fac[aa-bb],X-2)%X;
}
main()
{
    scanf("%lld%lld",&a,&b);
    if(b>=a)
    {
        printf("0");return 0;
    }
    fac[0]=1;
    for(i=1;i<=a+1;i++)fac[i]=fac[i-1]*i%X;
    for(i=b+1;i<=a;i++)
    {
        mans+=fac[i]*fac[a-i+1]%X*com(a+1,i+1)%X;
        mans%=X;
    }
    printf("%lld",mans);
}

• Subtask 1: 8 / 8
• Subtask 2: 0 / 92