HDU 5810 Balls and Boxes （找公式）

Balls and Boxes

题目链接：点我打开链接

Author

SYSU

Source

2016 Multi-University Training Contest 7

题意：给你n个球往m个盒子里扔，每个球扔进每个盒子的几率一样。问你：方差V的期望是多少。

我自己是猛怼规律，猛找公式的。。。

官方题解：

AC代码：

#include<bits/stdc++.h>#include<stdio.h>#include<string.h>#include<algorithm>#include<iostream>#include<cstring>#include<vector>#include<map>#include<queue>#include<set>#include<stack>using namespace std;typedef long long ll;typedef unsigned long long ull;int read(){int v = 0, f = 1;char c =getchar();while( c < 48 || 57 < c ){if(c=='-') f = -1;c = getchar();}while(48 <= c && c <= 57) v = v*10+c-48, c = getchar();return v*f;}int main(){ll n,m;ll A,B;while(~scanf("%I64d%I64d",&n,&m),n,m){A=n*(m-1);B=m*m;ll ans=__gcd(A,B);printf("%I64d/%I64d\n",A/ans,B/ans);}return 0;}