Balls and Boxes
题目链接：
http://acm.hdu.edu.cn/showproblem.php?pid=5810
Description
Mr. Chopsticks is interested in random phenomena, and he conducts an experiment to study randomness. In the experiment, he throws n balls into m boxes in such a manner that each ball has equal probability of going to each boxes. After the experiment, he calculated the statistical variance V as V = frac{sum_{i=1}^{m}(X_{i}-ar X)^{2}}{m} where Xi is the number of balls in the ith box, and X¯ is the average number of balls in a box. Your task is to find out the expected value of V.Input
The first line is an integer T (T <= 10), indicating the number of test case. Each test case begins with an integer N (1 <= N <= 1000), indicating the number of towns. Then N numbers in a line, the ith number ai (0 <= ai < N) has been described above.Output
For each test case, output "Case #X: Y" in a line (without quotes), where X is the case number starting from 1, and Y is "Yes" if you can construct successfully or "No" if it's impossible to reach the requirements. If Y is "Yes", output an integer M in a line, indicating the number of roads. Then M lines follow, each line contains two integers u and v (1 <= u, v <= N), separated with one single space, indicating a road direct from town u to town v. If there are multiple possible solutions, print any of them.Sample Input
3 3 2 1 0 2 1 1 4 3 1 1 0Sample Output
Case #1: Yes 2 1 2 2 3 Case #2: No Case #3: Yes 4 1 2 1 3 2 4 3 4Source
2016 Multi-University Training Contest 7##题意： 把n个球等可能地投入m个盒子. 求方差的期望.
##题解： 推了几个数，然后眼神拟合出公式..... ans = n*(m-1) / m^2 .
官方题解： 
##代码： ``` cpp #include