题目如下：
Problem I
23 Out of 5
Input: standard input
Output: standardoutput
Time Limit: 1 second
Memory Limit: 32 MB
Your task is to writea program that can decide whether you can find an arithmetic expression consistingof five given numbers
(1<=i<=5) that will yieldthe value 23.
For this problem we will only consider arithmetic expressions of the followingfrom:
where 
: {1,2,3,4,5} -> {1,2,3,4,5} is a bijective function
and 
{+,-,*} (1<=i<=4)
The Input consists of 5-Tupelsof positive Integers, each between 1 and 50.
Input is terminated by a line containing five zero‘s. This line should not beprocessed.
For each 5-Tupel print"Possible" (without quotes) if their exists an arithmetic expression(as described above) that yields 23. Otherwise print "Impossible".
1 1 1 1 1
1 2 3 4 5
2 3 5 7 11
0 0 0 0 0
Impossible
Possible
Possible
也是直接回溯就可以了，不过要注意π是个双射函数（bijective function)，所以要一一对应，不能重复使用，采用一个vis数组标记即可。
AC的代码如下：
UVA 23 out of 5,布布扣,bubuko.com
原文：http://blog.csdn.net/u013840081/article/details/37830501