UVA 23 out of 5

题目例如以下：

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)

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

也是直接回溯就能够了，只是要注意π是个双射函数（bijective function)。所以要一一相应。不能反复使用，採用一个vis数组标记就可以。

AC的代码例如以下：