1336：【例3-1】找树根和孩子

#include <bits/stdc++.h>
using namespace std;

int main()
{
    int n, m;
    cin >> n >> m;
    vector<int> dad(n + 1);
    vector <set<int>> son(n + 1);
    int x, y; // y是x的孩子
    for (int i = 0; i < m; i++) {
        cin >> x >> y;
        dad[y] = x;
        son[x].insert(y);
    }
    int root = x;
    while (dad[root] != 0) {
        root = dad[root];
    }
    int max = 1;
    for (int i = 2; i < son.size(); i++) {
        if (son[max].size() < son[i].size()) {
            max = i;
        }
    }
    cout << root << endl;
    cout << max << endl;
    for (set<int>::iterator it = son[max].begin();
         it != son[max].end(); ++it) {
        cout << *it << " ";
    }
    return 0;
}