hdu 1518 N根木棒 能否拼成正方形 dfs

 1 # include <cstdio>
 2 # include <cmath>
 3 # include <iostream>
 4 # include <cstring>
 5 # include <algorithm>
 6 using namespace std ;
 7 
 8 int n,sum,l;
 9 int a[30],vis[30];
10 int flag;
11 
12 bool cmp(int x , int y)
13 {
14     return x > y ;
15 }
16 
17 void dfs(int bian,int l,int k)  //已完成的边数  当前边的长度
18 {
19     int i;
20     if(bian==4)
21     {
22         flag = 1;
23         return ;
24     }
25     if(l == sum)
26     {
27         if(flag)
28             return ;
29         dfs(bian+1,0,0);
30     }
31     for(i = k; i<n; i++)
32     {
33         if(!vis[i] && l+a[i]<=sum)
34         {
35             if(flag)
36                 return;
37             vis[i] = 1;
38             dfs(bian,a[i]+l,i+1);
39             vis[i] = 0;
40         }
41     }
42 }
43 
44 int main()
45 {
46     int t;
47     scanf("%d",&t);
48     while(t--)
49     {
50         int i;
51         sum = 0;
52         scanf("%d",&n);
53         for(i = 0; i<n; i++)
54         {
55             scanf("%d",&a[i]);
56             sum+=a[i];
57         }
58         if(sum%4!=0)
59         {
60             printf("no\n");
61             continue;
62         }
63         sum = sum/4;
64 
65         for(i = 0; i<n; i++)
66         {
67             if(a[i]>sum)
68                 break;
69         }
70         if(i!=n)
71         {
72             printf("no\n");
73             continue;
74         }
75         sort(a,a+n,cmp) ;  //从大到小排序可以减小耗时
76 
77         memset(vis,0,sizeof(vis));
78         flag = 0;
79         dfs(0,0,0);
80         if(flag)
81             printf("yes\n");
82         else
83             printf("no\n");
84     }
85 
86     return 0;
87 }