Coleção de Upas

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

const int MAX = 200000; //2 * 10^5

vector<int> G[MAX];
bool cor[MAX];
int n, m;

int32_t main() {
    cin >> n >> m;
    while(m--) {
        int u, v;
        cin >> u >> v;
        G[u].push_back(v);
        G[v].push_back(u);
    }
    vector<int> ans;
    for(int i = n; i >= 1; i--) {
        bool fl = false;
        for(int &v : G[i])
            fl |= cor[v];
        if(!fl) {
            ans.push_back(i);
            cor[i] = 1;
        }
    }

    cout << ans.size() << '\n';
    for(int i = (int)ans.size() - 1; i >= 0; i--)
        cout << ans[i] << ' ';
    puts("");

    return 0;
}