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：