hdu1325 Is It A Tree? 基础并查集

 #include <stdio.h>
 #include <string.h>  

 int f[], g[];

 int find(int x)        //并查集的查找，找到共同的父亲
 {
     if (f[x] != x)
         f[x] = find(f[x]);
     return f[x];
 }

 int main()
 {
     int a, b, tt = ;
     while (scanf("%d%d", &a, &b) != EOF)
     {
         int i, flag = , t = ;
         if (a< && b<)
             break;
         for (i = ; i <= ; i++)
             f[i] = i;

         //合并
         if (find(a) == find(b))
             flag = ;
         else
             f[find(b)] = find(a);

         while (scanf("%d%d", &a, &b))
         {
             if (a ==  && b == )
                 break;
             int aa, bb;
             aa = find(a);
             bb = find(b);
             //合并
             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
                 flag = ;
             else
                 f[bb] = aa;
         }

         memset(g, , sizeof(g));
         for (i = ; i <= ; i++)    //保证只有一个根，不然就是森林了
         if (f[i] != i)
             g[find(i)]++;
         for (i = ; i <= ; i++)    //检查是否只有一个头结点就是了
         if (g[i]>)
             t++;
         if (t>)
             flag = ;
         if (flag == )
             printf("Case %d is not a tree.\n", tt++);
         else
             printf("Case %d is a tree.\n", tt++);
     }
     return ;
 }