DFS/四色定理/poj 1129 Channel Allocation

 1 #include<cstdio>
 2 #include<cstring>
 3 using namespace std;
 4 int n;
 5 int a[30][30];
 6 int c[30];
 7 
 8 bool pd(int x,int color)
 9 {
10     for (int i=1;i<x;i++)
11         if (a[x][i]==1 && c[i]==color) return false;
12     return true;
13 }
14 
15 bool dfs(int x,int color)
16 {
17     if (x>n) return true;
18     for (int i=1;i<=color;i++)
19         if (pd(x,i))
20         {
21             c[x]=i;
22             return dfs(x+1,color);
23         }
24     return false;
25 }
26 int main()
27 {
28     scanf("%d",&n);
29     while (n!=0)
30     {
31         memset(a,0,sizeof(a));
32         for (int i=1;i<=n;i++)
33         {
34             char str[30];
35             scanf("%s",str);
36             for (int j=2;j<strlen(str);j++)
37             {
38                 a[str[0]-‘A‘+1][str[j]-‘A‘+1]=1;
39                 a[str[j]-‘A‘+1][str[0]-‘A‘+1]=1;
40             }
41         }
42             if (dfs(1,1)) printf("1 channel needed.\n");
43             else
44             {
45                 for (int i=2;i<=4;i++)
46                     if (dfs(1,i))
47                     {
48                         printf("%d channels needed.\n",i);
49                         break;
50                     }
51             }
52         scanf("%d",&n);
53     }
54     return 0;
55 }