判空是否成树

 #include <bits/stdc++.h>
 #define MAX_N 100010
 #define INF 1<<28
 using namespace std;
 bool cnt[MAX_N];
 int edge,V;
 int u,v;
 int main()
 {
   while(scanf("%d%d",&u,&v) != EOF) {
     if (u == -1 && v == -1) break;
 
     if (u != 0 && v != 0) {
       memset(cnt,0,sizeof cnt);
       V = 0;
       edge = 0;
       V+=2;
       cnt[u] = cnt[v] = true;
       edge++;
       while (scanf("%d%d",&u,&v) != EOF && (u + v)) {
         if (!cnt[u]){
           V++;
           cnt[u] = true;
         }
         if (!cnt[v]){
           cnt[v]= true;
           V++;
         }
         edge++;
       }
       if (edge == V - 1){
          printf("Yes\n");
       } else {
         printf("No\n");
       }
     } else {
       printf("Yes\n");
     }
 
   }
   return 0;
 }