Shared code

#include<iostream>
#include<vector>
#include<stack>

#define REP(i, N) for(int i = 0;i < N;++i)
 
using namespace std;
 
int main() {
	int N, M;
	cin >> N >> M;
 
	vector< vector<int> > A(N + 1);
	vector<int> in(N + 1);
 
	int a, b;
 
	REP(i, M) {
		cin >> a >> b;
		A[a].push_back(b);
		++in[b];
	}

	stack<int> s;

	for (int i = 1;i < N + 1;++i) {
		if (!in[i]) s.push(i);
	}

	vector<int> topological;

	while (!s.empty()) {
		int u = s.top();
		s.pop();

		topological.push_back(u);

		for (int v : A[u]) {
			--in[v];

			if (!in[v]) s.push(v);
		}
	}

	if (topological.size() < N) {
		cout << "IMPOSSIBLE" << endl;
		return 0;
	}

	for (int a : topological) {
		cout << a << ' ';
	}
	cout << endl;
}