杭电 1518 Square （深搜）（回溯）

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

Square

Time Limit: 10000/5000 MS (Java/Others) Memory Limit: 65536/32768 K (Java/Others)
Total Submission(s): 8343 Accepted Submission(s): 2706

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

34 1 1 1 15 10 20 30 40 508 1 7 2 6 4 4 3 5

Sample Output

yesnoyes

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;  }

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