题解报告：hdu 1570 A C

题目链接：http://acm.hdu.edu.cn/showproblem.php?pid=1570

Problem Description

Are you excited when you see the title "AC" ? If the answer is YES , AC it ;
You must learn these two combination formulas in the school . If you have forgotten it , see the picture.



Now I will give you n and m , and your task is to calculate the answer .

Input

In the first line , there is a integer T indicates the number of test cases.
Then T cases follows in the T lines.
Each case contains a character 'A' or 'C', two integers represent n and m. (1<=n,m<=10)

Output

For each case , if the character is 'A' , calculate A(m,n),and if the character is 'C' , calculate C(m,n).
And print the answer in a single line.

Sample Input

2

A 10 10

C 4 2

Sample Output

3628800

6

解题思路：组合和全排列！（水题~）

AC代码：

#include<bits/stdc++.h>
using namespace std;
int main()
{
    int T,n,m,sum;
    char ch;
    cin>>T;
    while(T--){
        getchar();//吃掉回车符
        cin>>ch>>n>>m;
        if(m>n)swap(n,m);//保证m比n小
        sum=1;//计数
        if(ch=='A')for(int i=m;i>=1;i--)sum*=(n-i+1);//全排列
        if(ch=='C'){//组合
            if(n-m<m)m=n-m;//取m最小,减少计算
            for(int i=1;i<=m;i++)sum=sum*(n-i+1)/i;
        }
        cout<<sum<<endl;
    }
    return 0;
}