sgu222 分类： sgu 2015-06-12 09:27 24人阅读 评论(0) 收藏

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数学题，ans=C(k,n)2∗k!

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#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<iostream>
#include<algorithm>

int fact[20] = {1,1,2,6,24,120,720,5040,40320,362880,3628800};

int choice(int m, int n)
{
    return fact[n]/(fact[m]*fact[n-m]);
}
long long pow2(int x)
{
    return (long long)x*x;
}
int main()
{
    int n, k;

#ifndef ONLINE_JUDGE    
    freopen("sgu222.in","r",stdin);
    freopen("sgu222.out","w",stdout);
#endif

    std::cin >> n >> k;

    if(k > n) std::cout << 0 << std::endl;
    else std::cout << pow2(choice(k,n))*fact[k];

#ifndef ONLINE_JUDGE    
    fclose(stdin);
    fclose(stdout);
#endif
    return 0;
}

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