HDU1518：Square(DFS)

Problem Description

Given a set of sticks of various lengths, is it possible to join them end-to-end to form a square?

 

Input

The first line of input contains N, the number of test cases. Each test case begins with an integer 4 <= M <= 20, the number of sticks. M integers follow; each gives the length of a stick - an integer between 1 and 10,000.

 

Output

For each case, output a line containing "yes" if is is possible to form a square; otherwise output "no".

 

Sample Input

   
   
   
   

    
    
    
    3
4 1 1 1 1
5 10 20 30 40 50
8 1 7 2 6 4 4 3 5
   
   
   
   

 

Sample Output

   
   
   
   

    
    
    
    yes
no
yes
   
   
   
   

 

 

#include <stdio.h>
#include <string.h>
int n,sum,l;
int a[30],vis[30];
int flag;

void dfs(int bian,int l,int k)
{
    int i;
    if(bian==5)
    {
        flag = 1;
        return ;
    }
    if(l == sum)
    {
        dfs(bian+1,0,0);
        if(flag)
            return ;
    }
    for(i = k; i<n; i++)
    {
        if(!vis[i] && l+a[i]<=sum)
        {
            vis[i] = 1;
            dfs(bian,a[i]+l,i+1);
            if(flag)
                return;
            vis[i] = 0;
        }
    }
}

int main()
{
    int t;
    scanf("%d",&t);
    while(t--)
    {
        int i;
        sum = 0;
        scanf("%d",&n);
        for(i = 0; i<n; i++)
        {
            scanf("%d",&a[i]);
            sum+=a[i];
        }
        if(sum%4!=0)
        {
            printf("no\n");
            continue;
        }
        sum = sum/4;
        for(i = 0; i<n; i++)
        {
            if(a[i]>sum)
                break;
        }
        if(i!=n)
        {
            printf("no\n");
            continue;
        }
        memset(vis,0,sizeof(vis));
        flag = 0;
        dfs(1,0,0);
        if(flag)
            printf("yes\n");
        else
            printf("no\n");
    }

    return 0;
}