并查集。需要考虑入度。

1 #include 
     
       2 #include 
      
        3  4 #define MAXNUM 10005 5  6 int bin[MAXNUM]; 7 int degree[MAXNUM]; 8 int nums[MAXNUM]; 9 10 int find(int x) {11     int r = x;12 13     while (bin[r] != r)14         r = bin[r];15 16     return r;17 }18 19 int main() {20     int x, y, fx, fy, n, case_n = 0;21     int i, flg;22 23     while (1) {24         scanf("%d %d", &x, &y);25         if (x<0 && y<0)26             break;27         memset(degree, 0, sizeof(degree));28         n = 0;29         ++case_n;30         if (x==0 && y==0) {31             printf("Case %d is a tree.\n", case_n);32             continue;33         }34         for (i=0; i
       
         1) {70                 flg = 0;71                 break;72             }73         }74         if (flg)75             printf("Case %d is a tree.\n", case_n);76         else77             printf("Case %d is not a tree.\n", case_n);78     }79 80     return 0;81 }