CodeForces 452C Magic Trick (排列组合)

#include <iostream>
#include <cstdio>
#include<cmath>
#include<algorithm>
using namespace std;
double num[1000000];
void init_arrary(){
    for (int i = 1; i <= 1000000; ++i)
    {
        num[i] = num[i - 1] + log(i);
    }
}
double Cn(int m,int n){
    return num[m] - num[n] - num[m-n];
}
int main(){
    double n , m;
    int i,j;
    init_arrary();
    while(cin >> n >> m){
        double sum=0,res;
            res=0;
            for(i=1;i<=min(m,n);i++){
                sum=Cn(m,i)+Cn(n*m-m,n-i)-Cn(n*m,n);
                sum=exp(sum);
                 res+=sum*i*i/n;//期望吧
            }
            cout<<res<<endl;
    }
    return 0;
}