
ZOJ Problem Set  2522
To enforce communication, n cities decided to exchange citizens for some time. There are ai (1<=i<=n, ai>0) families in the ith city join in this activity, which goes like this: each family will send one child to live with another family for one week, and obviously will accommodate one. However the children will not be sent to the families in their own cities. Then how many ways of arrangement can be made? Input There are several test cases. Each test case contains two lines.
The first line there is an integer n, the number of cities. Output: For each test case, print in one line the number of permutation. Sample Input: 1 15 3 2 2 3 Sample Output: 0 288 Author: YANG, Xiao Source: ZOJ Monthly, September 2005 