水题，判断是否是一棵树，如果一个结点有多个父亲or图中有环就不是。
1 #include <stdio.h> 2 #include <string.h> 3 int fa[1005],v[1005]; 4 int Find(int n) 5 { 6 if(fa[n] == -1) 7 return n; 8 fa[n] = Find(fa[n]); 9 return fa[n]; 10 } 11 int main() 12 { 13 int ca=1,a,b,i,j,f; 14 bool ok; 15 while(scanf("%d%d",&a,&b)) 16 { 17 if(a==-1 && b==-1) break; 18 if(!a && !b) 19 { 20 printf("Case %d is a tree.\n",ca++); 21 continue; 22 } 23 ok = 1; i = 0; 24 memset(fa,-1,sizeof fa); 25 fa[b] = a; v[i] = b; 26 if(a == b) 27 { 28 ok = 0; 29 printf("Case %d is not a tree.\n",ca++); 30 } 31 while(scanf("%d%d",&a,&v[++i]),a&&v[i]) 32 if(ok) 33 { 34 if(fa[v[i]] != -1 || Find(a)==v[i]) 35 { 36 ok = 0; 37 printf("Case %d is not a tree.\n",ca++); 38 } 39 fa[v[i]] = a; 40 } 41 if(ok) 42 { 43 f = Find(v[0]); 44 for(j = 1; j <= i&&Find(v[j]) == f; j++); 45 if(j < i) 46 printf("Case %d is not a tree.\n",ca++); 47 else printf("Case %d is a tree.\n",ca++); 48 } 49 } 50 return 0; 51 }