1 #include <stdio.h>
2 #include <string.h>
3
4 int f[1001], g[1001];
5
6 int find(int x) //并查集的查找，找到共同的父亲
7 {
8 if (f[x] != x)
9 f[x] = find(f[x]);
10 return f[x];
11 }
12
13 int main()
14 {
15 int a, b, tt = 1;
16 while (scanf("%d%d", &a, &b) != EOF)
17 {
18 int i, flag = 0, t = 0;
19 if (a<0 && b<0)
20 break;
21 for (i = 0; i <= 1000; i++)
22 f[i] = i;
23
24 //合并
25 if (find(a) == find(b))
26 flag = 1;
27 else
28 f[find(b)] = find(a);
29
30 while (scanf("%d%d", &a, &b))
31 {
32 if (a == 0 && b == 0)
33 break;
34 int aa, bb;
35 aa = find(a);
36 bb = find(b);
37 //合并
38 if (aa == bb || bb != b) //成环就不是树，后面的是检查是否有两个父节点
39 flag = 1;
40 else
41 f[bb] = aa;
42 }
43
44 memset(g, 0, sizeof(g));
45 for (i = 1; i <= 1000; i++) //保证只有一个根，不然就是森林了
46 if (f[i] != i)
47 g[find(i)]++;
48 for (i = 1; i <= 1000; i++) //检查是否只有一个头结点就是了
49 if (g[i]>0)
50 t++;
51 if (t>1)
52 flag = 1;
53 if (flag == 1)
54 printf("Case %d is not a tree.\n", tt++);
55 else
56 printf("Case %d is a tree.\n", tt++);
57 }
58 return 0;
59 }
