Eddy's AC难题 HDU

http://acm.hdu.edu.cn/showproblem.php?pid=2200

#include<cstdio>#include<cstring>using namespace std;typedef long long ll;ll cal(ll n,ll m){ll sum=1;for(ll i=1;i<=m;i++){sum=sum*(n-i+1)/i;//计算Cnm的套路，一个是整数会有分数，导致出现0 .竟然 sum*= 都不可以 //printf("n:%lld m:%lld sum:%lld i:%lld\n",n,m,sum,i);}return sum;}ll ans(ll n){ll sum=0;for(ll i=2;i<=n;i++){sum+=(i-1)*cal(n,i);}return sum;}int main(){ll n;while(~scanf("%lld",&n)){printf("%lld\n",ans(n));}return 0;}