UVA - 10375 Choose and divide

m!
C(m,n) = --------
         n!(m-n)!
例如这个，先找到n与(m-n)之间的较小值，然后式子就等于(p*...*(p-Min+1))/Max
接下来就是简单的除法了
#include <iostream>
#include <cstdio>
#include <cstring>
#include <cmath>
using namespace std;

double cal(int p,int q,int r,int s){
    double cnt = 1;
    q = min(q,p-q);
    s = min(s,r-s);
    p = p-q+1,r=r-s+1;
    int i = 1,j = 1;
    while (i <= q || j <= s){
        if (i <= q){
            cnt = cnt*p/i;
            p++,i++;
        }
        if (j <= s){
            cnt = cnt/r*j;
            r++,j++;
        }
    }
    return cnt;
}

int main(){
    int p,q,r,s;
    while (scanf("%d%d%d%d",&p,&q,&r,&s) != EOF){
        double ans = cal(p,q,r,s);
        printf("%.5lf\n",ans);
    }
    return 0;
}