题解 | #Is It A Tree?#

#include<iostream>
#include<map>
using namespace std;
int main()
{
    int s, e, ss = 0, cnt;
    bool ans = true;
    map<int, int> mp;   //s --> e
    while (cin >> s >> e) {
        if (s == 0 && e == 0) {
            cnt = 0;
            ss++;
            for (auto it : mp) {
                if (it.second == 0) {
                    cnt += 1;
                    if (cnt > 1) {
                        ans = false;
                        break;
                    }
                }
                if (it.second > 1) {
                    ans = false;
                    break;
                }
            }
            if(cnt == 0 && mp.size() == 0)
                ans = true;
            else if(cnt == 0)
                ans = false;
            cout << "Case " << ss << " is " << (ans == false? "not " : "") << "a tree." << endl;
            mp.clear();
            ans = true;
        }
        else {
            if (mp.find(e) == mp.end())
                mp[e] = 1;
            else
                mp[e] += 1;
            if (mp.find(s) == mp.end())
                mp[s] = 0;
            if (s == e)
                ans = false;
        }
    }
}