http://acm.hdu.edu.cn/showproblem.php?pid=1518

Square

Time Limit: 10000/5000 MS (Java/Others)    Memory Limit: 65536/32768 K (Java/Others)

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

AC代码：

#include<iostream>
#include<cstring>
using namespace std;  

int v[23],n,flag;
int a[23],sum;  

void dfs(int p,int ans,int t)
{
	if(ans==sum/4)
	{
		p++;
	//	cout<<p<<"-------\n";
		if(p==4)
		{
			flag=1;
			return ;
		}
		else ans=0;
		t=0;
	}  

	if(flag)   return ;
	for(int i=t;i<n;i++)
		if(!v[i] && a[i]+ans<=sum/4)
		{
			v[i]=1;
		//	cout<<ans+a[i]<<"   ";
			dfs(p,ans+a[i],i);
			v[i]=0;
		}
}  

int main()
{
	int T;
	cin>>T;
	while(T--)
	{
		cin>>n;
		int max;
		max=sum=0;
		for(int i=0;i<n;i++)
		{
			cin>>a[i];
			sum += a[i];
			if(max<a[i])   max=a[i];
		}  

		if(sum%4!=0 || max>sum/4)
			cout<<"no\n";
		else
		{
			memset(v,0,sizeof(v));
			flag=0;
			dfs(0,0,0);
			if(flag)   cout<<"yes\n";
			else     cout<<"no\n";
		}
	}  

	return 0;
}

杭电 1518 Square （深搜）（回溯）,布布扣,bubuko.com