hdu 1518 Square（深搜+剪枝）

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

题目大意:根据题目所给的几条边，来判断是否能构成正方形，一个很好的深搜应用，注意剪枝，以防超时！

 1 #include <iostream>
 2 #include <cstdio>
 3 #include<algorithm>
 4 #include <cstring>
 5 using namespace std;
 6 int ap[30],visit[30];
 7 int l,n;
 8 int dfs(int len,int gen,int iqq)
 9 {
10     if (gen==3)
11         return 1;
12     for(int i=iqq; i<n; i++)
13     {
14         //cout<<visit[len]<<endl;
15         if (!visit[i])
16         {
17             visit[i]=1;
18             if (len+ap[i]==l)
19             {
20                 //cout<<len<<endl;
21                 if(dfs(0,gen+1,0))
22                     return 1;
23             }
24             else if (len+ap[i]<l)
25             {
26                 //cout<<len<<endl;
27                 if(dfs(len+ap[i],gen,i)) return 1;
28             }
29             visit[i]=0;
30         }
31     }
32     return 0;
33 }
34 int main ()
35 {
36     int t,sum;
37     while (cin>>t)
38     {
39         while (t--)
40         {
41             cin>>n;
42             sum=0;
43             //Max=0;
44             for (int i=0; i<n; i++)
45             {
46                 cin>>ap[i];
47                 sum+=ap[i];
48             }
49             memset(visit,0,sizeof(visit));
50             sort(ap,ap+n);
51             if (sum%4==0&&n>=4&&ap[n-1]<=sum/4)
52             {
53 
54                 l=sum/4;
55                 if (dfs(0,0,0))
56                     printf ("yes
");
57                 else
58                     printf ("no
");
59                 //cout<<n<<endl;
60             }
61             else printf ("no
");
62         }
63     }
64 }