All You Need to Know About Breadth First Search Algorithm

From searching for restaurants and hotels nearby to communicating peer-to-peer, graph traversal methods are used almost everywhere.

This concept is quite fascinating for programmers as well as interviewers and this is why it is often asked in interviews.

BFS graph Traversal method is applied in many real-life problems. This makes it an important topic or concept which every programmer must be aware of.

However, if you are just starting to code or you are revising your basics, this post will be helpful to learn what breadth-first search and how it works.

We’ll talk about the bread first search algorithm in detail to help you get a clearer picture of the topic.

What Is Graph Traversal?

Before understanding the BFS graph algorithm, let’s first understand what graph traversal is.

The phenomenon of exploring and visiting each node of the graph is known as graph traversal. Simply said, you need to visit each edge and vertex of a graph in a way that you explore each vertex once only.

Though it may sound simple this is not it!

Different approaches are available t for graph traversal. In that case, choosing a technique that is best suited for the given problem is a tough nut to crack.

Therefore, understanding how each algorithm works can be of great help. With that said, let’s begin with the breadth-first search algorithm.

Introduction To Breadth-First Algorithm

In the breadth-first traversing algorithm, a random node is selected initially. You need to then traverse the whole graph layer-wise in a way that all the nodes and their children nodes are traversed once only.

While working with the BFS graph algorithm you will come across two types of nodes. They are as follows:

Exploring Node: This is the node adjacent to the initial node that you have selected. This is the node that you will have to explore next.
Visiting Node: This is the node that you will select to visit and start the traversal process.

BFS Algorithm

By now you must have an idea of what BFS graph traversal is. But do you have an idea how it works? If not, let’s understand its algorithm.

Set status= 1 for each node of G
Enqueue the beginning node and set the status as 2
Dequeue N and set the status as 3 after processing it.
Enqueue every neighboring node which has a status as 1
Set their status as 2
Repeat the same set of steps until the queue is empty.

So, this is how the BFS graph traversal works. But how to write the code?

Let’s check out the pseudocode in the next section!

BFS Graph Traversal Pseudocode

You are given that S is the source node.

Breadth_first(G, S)

Let K be the queue

K.enqueue(S)

Assign S as visited

While (K is not null)

R= k.dequeue()

For all the neighbors q of R in the Graph G

If q is not visited

K.enqueue(q)

Assign q as visited

In this pseudocode, we have followed the following steps:

In the first step, (G, S) is the given input. Here, S is the source node and G is the graph that we are traversing.
Next, a queue K needs to be created and should be initialized with the source node S
Now, mark all the child nodes of S
Extract the S node from the Queue and then visit its child nodes.
You will now have to check all the child nodes of R
Store q (child nodes) in K to visit the child nodes.
Repeat the same till K is empty.

Applications Of Breadth-First Search

So now that you have understood what is BFS graph traversal algorithm is and how it works, you must be wondering where you can use it. Here, we have come up with some applications of the breadth-first search algorithm.

Search Engine Crawlers: One of the most common applications of breadth-first search is search engine crawlers. This algorithm is used to index all the pages of a website. The algorithm will consider the source page as the root node and then considers all the pages as the exploring nodes. In this way, it crawls all the pages of a website.
GPS System: Another common application of the algorithm which we use daily in our lives is GPS systems. In GPS systems, the breadth-first search algorithm is used to find neighboring locations.
Peer To Peer Networking: In a peer-to-peer network, this traversal method can be used to find the neighboring nodes and communication devices. BitTorrent is one example of such peer-to-peer communication.
Social Networking Sites: Many networking websites search for people near you. These websites also use the breadth-first search algorithm to find a person within a distance “k” and till k levels.
Garbage Collection: Another application of the breadth-first search algorithm is garbage collection. The algorithm uses Cheney’s algorithm to collect garbage values.
Subarray With 0 Sum: Subarray with 0 sum is one common problem that you can resolve using the algorithm. You need to find a pair of elements whose sum is 0 in an array.

Advantages Of Breadth-First Search

Why should you choose the breadth-first search algorithm over any other traversal methods? If you are not sure of the reasons, Here’s why.

With this method, it is certain that you will find the solution if there exists a solution.
BFS graph traversal will never end up being on unwanted nodes.
In case there are multiple possible solutions, the BFS method will find the solution with minimum steps.

Disadvantages Of Breadth-First Search

Despite being an effective traversal method, the breadth-first search algorithm is still not the best option in every case. There are some disadvantages of the breadth-first search algorithm which you must know before using the method.

The major issue with the BFS graph traversal is its memory constraints. This is because this algorithm stores every node of the current level to go to the next level.
In case the solution to the required problem is far away, the algorithm may take a lot of time to find the solution.

Conclusion

Breadth-first search is one of the common traversal methods which is used in various programming concepts and your real life. Navigation systems, garbage collection, search engine crawlers, and peer-to-peer networking are some common applications where you will find BFS graph traversal methods.

Above we have explained in detail what a breadth-first search algorithm is and how you can implement it.

Go through the complete algorithm and pseudocode to get a better hold of the concept.

Also Read – Web App Development With Node.js: 5 Frameworks To Know