历年上机题----Is It A Tree?

https://www.nowcoder.com/practice/1c5fd2e69e534cdcaba17083a5c56335?tpId=61&tqId=29506&tPage=1&ru=/kaoyan/retest/1002&qru=/ta/pku-kaoyan/question-ranking

保证是一棵树，需要满足以下条件：

1. 只有一个根节点

2. 每个节点只能有一个父亲节点

3. 不能有环

#include <bits/stdc++.h>
using namespace std;
const int maxn=1e5+7;
int main()
{
    #ifndef ONLINE_JUDGE
        freopen("in.txt","r",stdin);
        freopen("out.txt","w",stdout);
    #endif
    int a,b;
    int T=1;
    int f[maxn];
    set<int> S;
    memset(f,0,sizeof(f));
    while(scanf("%d%d",&a,&b)!=EOF)
    {
        if(a==-1&&b==-1) break;
        if(a==0&&b==0)
        {
            if(S.size()==0)
            {
                cout<<"Case "<<T<<" is a tree."<<endl;   
            }
            else
            {
                bool flag=0;
                int cnt=0;
                for(auto t:S)
                {
                    if(f[t]>1)
                    {
                        flag=1;
                        break;
                    }
                    if(f[t]==0)
                    {
                        cnt++;
                    }
                }
                if(cnt>1||cnt==0) flag=1;
                if(flag)
                {
                    cout<<"Case "<<T<<" is not a tree."<<endl;
                }
                else
                {
                    cout<<"Case "<<T<<" is a tree."<<endl;   
                }
            }
            T++;
            memset(f,0,sizeof(f));
            S.clear();
        }
        else
        {
            S.insert(a);
            S.insert(b);
            f[b]++;
           // cout<<a<<" "<<b<<endl;
            if(a==b&&a!=0) f[a]++;        
        }
    
    }
    return 0;
}