A simple graph traversal example: using bfs

main.cpp

#include <iostream>
#include <vector>
#include <queue>
using namespace std;

// Sample input
// 4 4
// 0 1
// 0 2
// 1 3
// 2 3
// 0

// Graph datastructure -> Adjecency list
vector<vector<int>> g;

int main() {
  freopen("graph.in", "r", stdin);

  int n, m;  // number of vertices and edges
  cin >> n >> m;
  g.resize(n);

  for(int i = 0; i < m; ++i) {
    int u, v; cin >> u >> v;
    // suppose that the graph is undirected
    g[u].push_back(v);
    // This should be omited if the graph is directed.
    g[v].push_back(u);
  }
  // node from where to start the discovery
  int start; cin >> start; 

  queue<int> q;
  vector<bool> seen(n, false);
  q.push(start);
  seen[start] = true;
  while(!q.empty()) {
    int u = q.front(); q.pop(); 
    cout << "visiting " << u + 1<< endl;
    for(int v: g[u]) {
      if(seen[v]) {
        continue;
      }
      seen[v] = true;
      q.push(v);
    }   
  } 
}