HDU 1325 Is It A Tree?

 1 #include <stdio.h>
 2 #include <string.h>
 3 #define MAXN    (1002)
 4 
 5 int    set[MAXN];
 6 int    flag, tcase;
 7 
 8 void init()
 9 {
10     int i;
11     memset(set, -1, sizeof(set));
12     flag = 0;
13 }
14 
15 int find(int x)
16 {
17     if (set[x] == -1)
18         set[x] = x;
19     if (set[x] == x)
20         return x;
21     return set[x] = find(set[x]);
22 }
23 
24 int main()
25 {
26     int i, j;
27     int x, y;
28     tcase = 1;
29     init();
30     while (scanf("%d %d", &x, &y) != EOF)
31     {
32         if (x == -1 && y == -1)
33             break;
34         if (x == 0 && y == 0)
35         {
36             printf ("Case %d", tcase++);
37             if (!flag)
38             {
39                 j = 0;
40                 for (i = 0; i < MAXN; ++i)
41                     if (set[i] == i)
42                         ++j;
43                     if (j > 1)    // not a tree: 1 2 3 4 0 0, a tree: 0 0
44                         flag = 1;
45             }
46             if (!flag)
47             {
48                 printf (" is a tree.\n");
49             }
50             else
51                 printf (" is not a tree.\n");
52             init();
53             continue;
54         }
55         int px = find(x);
56         int py = find(y);
57 
58         if (py != y && px != py || x == y || (x != y && px == py))// not a tree: 1 2 1 3 2 1 0 0
59         {// not a tree: 1 1 0 0
60             flag = 1;
61         }
62         else
63             set[py] = set[px];
64     }
65     return 0;
66 }