bfs.cpp

// A Quick implementation of BFS using
// vectors and queue
#include <bits/stdc++.h>
#define pb push_back

using namespace std;

vector<bool> v;
vector<vector<int> > g;

void edge(int a, int b)
{
	g[a].pb(b);

	// for undirected graph add this line
	// g[b].pb(a);
}

void bfs(int u)
{
	queue<int> q;

	q.push(u);
	v[u] = true;

	while (!q.empty()) {

		int f = q.front();
		q.pop();

		cout << f << " ";

		// Enqueue all adjacent of f and mark them visited
		for (auto i = g[f].begin(); i != g[f].end(); i++) {
			if (!v[*i]) {
				q.push(*i);
				v[*i] = true;
			}
		}
	}
}

// Driver code
int main()
{
	int n, e;
	cin >> n >> e;

	v.assign(n, false);
	g.assign(n, vector<int>());

	int a, b;
	for (int i = 0; i < e; i++) {
		cin >> a >> b;
		edge(a, b);
	}

	for (int i = 0; i < n; i++) {
		if (!v[i])
			bfs(i);
	}

	return 0;
}