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.
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where: {1,2,3,4,5} -> {1,2,3,4,5} is a bijective function
and{+,-,*} (1<=i<=4)
Input
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.
Output
For each 5-Tupel print"Possible" (without quotes) if their exists an arithmetic expression(as described above) that yields 23. Otherwise print "Impossible".
Sample Input
1 1 1 1 1
1 2 3 4 5
2 3 5 7 11
0 0 0 0 0
Sample Output
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：