Hdu 5379 Mahjong tree (dfs + 组合数)

 1 #include <queue>
 2 #include <cstdio>
 3 #include <cstring>
 4 #include <iostream>
 5 #include <algorithm>
 6 #pragma comment (linker, "/STACK:1024000000,1024000000")
 7 using namespace std;
 8 
 9 const int maxn = 100005;
10 const int mod = (1e9 + 7);
11 typedef long long LL;
12 struct node
13 {
14     int to, next;
15 }edge[maxn*2];
16 int head[2][maxn], vis[maxn], tot, flag;
17 LL res;
18 
19 void Add (int from, int to)
20 {
21     edge[tot].to = to;
22     edge[tot].next = head[0][from];
23     head[0][from] = tot ++;
24     head[1][from] ++;
25 }
26 
27 void dfs (int u)
28 {
29     int k, cnt;
30     k = cnt = 0;
31     vis[u] = 1;
32     for (int i=head[0][u]; i!=-1; i=edge[i].next)
33     {
34 
35         int v = edge[i].to;
36         if (vis[v])
37             continue;
38         if (!vis[v] && head[1][v]==1)
39             cnt ++;
40         else
41             dfs (v);
42         k ++;
43     }
44     if (k - cnt > 2)
45     {
46         res = 0;
47         return;
48     }
49     if (k != cnt)
50         res = (res * 2) % mod;
51     while (cnt)
52     {
53         res = (res * cnt) % mod;
54         cnt --;
55     }
56 }
57 int main ()
58 {
59     int t, n, l = 0;
60     scanf ("%d", &t);
61 
62     while (t --)
63     {
64         scanf ("%d", &n);
65         memset (head[0], -1, sizeof(head[0]));
66         memset (head[1], 0, sizeof(head[1]));
67         memset (vis, 0, sizeof(vis));
68         tot = flag = 0;
69 
70        for (int i=1; i<n; i++)
71         {
72             int u, v;
73             scanf ("%d %d", &u, &v);
74             Add (u, v);
75             Add (v, u);
76         }
77         res = 2;
78         dfs (1);
79         printf ("Case #%d: %lld\n", ++l, n==1?1:res);
80     }
81     return 0;
82 }