检查有向图是否是树

 1 #include <iostream>
 2 #include <cstring>
 3 #include <queue>
 4 #include <set>
 5 #include <map>
 6 using namespace std;
 7 const int maxn=1e5+5;
 8 
 9 int x;
10 int f[maxn];
11 int find(int x){
12     if(x!=f[x]){
13         f[x]=find(f[x]);
14     }
15     return f[x];
16 }
17 void init(){
18     for(int i=0;i<=100000;i++)    
19         f[i]=i;
20 }
21 map<int,int>M;
22 set<int> S;
23 int main(){
24     int a,b;int flag=1;
25     std::ios::sync_with_stdio(false);
26     init();
27     int cas=1; 
28     while(cin>>a>>b){
29         if(a==-1&&b==-1) break;
30         if(a==0&&b==0)
31         {
32             //cout <<"flag="<<flag<<endl;
33             //cout <<"S.ize"<<S.size()<<endl;
34             if(flag==0) 
35             {
36                 cout <<"Case "<<cas++<<" is not a tree.\n";
37                 flag=1;
38             }
39             else 
40             {
41                 //cout <<"in";
42                 int sign;set<int>::iterator it=S.begin();
43                 if(!S.empty()) sign=find(*it),it++;
44                 for(;it!=S.end();it++){
45                     if(find(*it)!=sign) flag=0;
46                     //cout <<"in";
47                 }
48                 
49                 if(flag) 
50                 {
51                     int num=0;
52                     map<int,int>:: iterator iter=M.begin();
53                     //cout <<M.size()<<endl;
54                     for(;iter!=M.end();iter++){
55                         //cout <<iter->first<<" "<<iter->second<<endl;
56                         if(iter->second==0) num++;
57                     }
58                     if(num>1) cout <<"Case "<<cas++<<" is not a tree.\n";
59                         else cout <<"Case "<<cas++<<" is a tree.\n";
60                 }
61                     else cout <<"Case "<<cas++<<" is not a tree.\n";
62             }
63         //cout <<"in";
64             flag=1;
65             S.clear();
66             M.clear();
67             init();
68         }
69         else
70         {
71             S.insert(a);S.insert(b); 
72             M[b]=1;if(!M.count(a)) M[a]=0;
73             int fa=find(a);
74             int fb=find(b);
75             //cout <<"root"<<fa<<" "<<fb<<endl;
76             if(fa==fb)
77             {
78                 flag=0;
79             }else
80             {
81                 f[fb]=fa;
82             }
83         }
84     }
85     
86     return 0;
87 }