# BFS using STL for competitive coding

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 multple BFSs possible for a graph (even from a particular vertex).

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)` `{` ` ` `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;` `}` |

Input: 8 10 0 1 0 2 0 3 0 4 1 5 2 5 3 6 4 6 5 7 6 7 Output: 0 1 2 3 4 5 6 7

This article is contributed by **Nikhil Mahendran**. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

Attention reader! Don’t stop learning now. Get hold of all the important DSA concepts with the **DSA Self Paced Course** at a student-friendly price and become industry ready. To complete your preparation from learning a language to DS Algo and many more, please refer **Complete Interview Preparation Course****.**

In case you wish to attend **live classes **with experts, please refer **DSA Live Classes for Working Professionals **and **Competitive Programming Live for Students**.