【noi 2.5_1789】算24（dfs）

 1 #include<cstdio>
 2 #include<cstdlib>
 3 #include<cstring>
 4 #include<algorithm>
 5 #include<iostream>
 6 using namespace std;
 7 
 8 const double eps=0.00000001;
 9 double a[4];
10 int v[4];
11 
12 double mabs(double x)
13 {   return x>0?x:-x;   }
14 
15 int dfs(int x)
16 {
17     if (x==3)
18     {
19       for (int i=0;i<4;i++)
20         if (!v[i] && mabs(a[i]-24.0)<eps) return 1;
21       return 0;
22     }
23     /*for (int i=0;i<4;i++)
24      if (!v[i])
25       for (int j=0;j<4;j++)
26        if (!v[j]&&i!=j)
27        {
28          v[j]=1;
29          double tmp=a[i],tmpp=a[j];
30          
31          a[i]=tmp+tmpp;
32          if (dfs(x+1)) return 1;
33          a[i]=tmp-tmpp;
34          if (dfs(x+1)) return 1;
35          a[i]=tmpp*tmp;
36          if (dfs(x+1)) return 1;
37          a[i]=tmpp/tmp;
38          if (dfs(x+1)) return 1;
39          
40          a[i]=tmp;
41          v[j]=0;
42        }*///244ms
43     for (int i=0;i<4;i++)
44      if (!v[i])
45       for (int j=i+1;j<4;j++)//区别
46        if (!v[j])
47        {
48          v[j]=1;
49          double tmp=a[i],tmpp=a[j];
50          
51          a[i]=tmp+tmpp;
52          if (dfs(x+1)) return 1;
53          a[i]=tmp-tmpp;
54          if (dfs(x+1)) return 1;
55          a[i]=tmpp-tmp;//
56          if (dfs(x+1)) return 1;
57          a[i]=tmpp*tmp;
58          if (dfs(x+1)) return 1;
59          a[i]=tmpp/tmp;
60          if (dfs(x+1)) return 1;
61          a[i]=tmp/tmpp;//
62          if (dfs(x+1)) return 1;
63          
64          a[i]=tmp;
65          v[j]=0;
66        }//102ms
67     return 0;
68 }
69 
70 int main()
71 {
72     while (1)
73     {
74       for (int i=0;i<4;i++) scanf("%lf",&a[i]);
75       if (!a[0]) break;
76       memset(v,0,sizeof(v));
77       if (dfs(0)) printf("YES\n");
78       else printf("NO\n");
79     }
80     return 0;
81 }