23 Out of 5 uva+回溯

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

解决方案：搞了半天，原来顺序可以换的。

#include<iostream>#include<cstdio>#include<cstring>using namespace std;int a[5];bool vis[5];bool flag;void dfs(int s,int sum){   if(s==5){    if(sum==23) flag=true;    return ;   }   for(int i=0;i<5;i++){    if(!vis[i]){            vis[i]=true;    if(s==0)    {dfs(s+1,a[i]);}    else {dfs(s+1,sum-a[i]);    dfs(s+1,sum*a[i]);    dfs(s+1,sum+a[i]);}    vis[i]=false;    }   }}int main(){   while(~scanf("%d%d%d%d%d",&a[0],&a[1],&a[2],&a[3],&a[4])&&(a[0]+a[1]+a[2]+a[3]+a[4])){    flag=false;    memset(vis,false,sizeof(vis));    dfs(0,0);    if(flag) cout<<"Possible\n";    else cout<<"Impossible\n";   }   return 0;}