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BFS using STL for competitive coding

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  • Difficulty Level : Easy
  • Last Updated : 15 Nov, 2021

A STL based simple implementation of BFS using queue and vector in STL. The adjacency list is represented using vectors of vector. 

In BFS, we start with a node.
1) Create a queue and enqueue source into it. 
   Mark source as visited.
2) While queue is not empty, do following
    a) Dequeue a vertex from queue. Let this 
       be f.
    b) Print f
    c) Enqueue all not yet visited adjacent
       of f and mark them visited.

Below is an example BFS starting from source vertex 1. Note that there can be multiple BFSs possible for a graph (even from a particular vertex). 

BFS using STL for competitive coding

For more details of BFS, refer this post
The code here is simplified such that it could be used in competitive coding. 


// 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)
    // for undirected graph add this line
    // g[b].pb(a);
void bfs(int u)
    queue<int> q;
    v[u] = true;
    while (!q.empty()) {
        int f = q.front();
        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]) {
                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])
    return 0;

8 10
0 1
0 2
0 3
0 4
1 5
2 5
3 6
4 6
5 7
6 7

0 1 2 3 4 5 6 7

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