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BFS using STL for competitive coding
• Difficulty Level : Easy
• Last Updated : 20 Oct, 2017

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 ``#define pb push_back`` ` `using` `namespace` `std;`` ` `vector<``bool``> v;``vector > 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.

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