Breadth First Search

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November 1, 2022

Coding & Algorithm Interview 


Breadth-First Search (BFS) is a fundamental graph traversal algorithm used to explore and search for nodes or vertices in a graph in a breadthward motion. It is a simple and intuitive algorithm that operates by visiting all the vertices at the current level before moving on to the next level. BFS is widely used in various applications, such as web crawling, social network analysis, shortest path finding, and more. In this tech blog, we will delve deep into the intricacies of BFS, exploring its key concepts, algorithmic details, time and space complexity analysis, and practical implementation tips.

Key Concepts of Breadth-First Search

  1. Graph: A graph is a data structure that represents a collection of vertices or nodes connected by edges or links. It can be represented as an adjacency list or an adjacency matrix.
  2. Traversal: Traversal refers to the process of visiting all the vertices or nodes of a graph in a systematic manner. BFS is one of the graph traversal techniques that uses a breadthward approach.
  3. Queue: A queue is a data structure that follows the First-In-First-Out (FIFO) principle, where elements are inserted at the rear and removed from the front. BFS uses a queue to keep track of the vertices to be visited.

Algorithmic Details of Breadth-First Search

The BFS algorithm can be summarized in the following steps:

  1. Choose a starting vertex as the root of the traversal and mark it as visited.
  2. Enqueue the root vertex into a queue.
  3. While the queue is not empty, do the following: a. Dequeue a vertex from the front of the queue. b. Visit the dequeued vertex and process it as needed. c. Enqueue all the unvisited neighbours of the dequeued vertex into the queue and mark them as visited.
  4. Repeat step 3 until the queue becomes empty, indicating that all vertices have been visited.

It’s important to note that BFS guarantees the shortest path to the visited vertices from the root vertex because it explores the graph level by level.

Time and Space Complexity Analysis: The time complexity of BFS is O(V + E), where V is the number of vertices and E is the number of edges in the graph. This is because BFS visits each vertex and each edge exactly once. The space complexity of BFS is O(V), as the worst-case scenario would be storing all the vertices in the queue at once.

Practical Implementation Tips

  1. Choose the right data structure: BFS requires a queue to keep track of vertices to visit. Depending on the programming language and specific use case, you can implement the queue using built-in data structures like lists or arrays, or use specialized data structures like queues or dequeues.
  2. Use graph representations wisely: Graphs can be represented in various ways, such as adjacency lists or adjacency matrices. Choose the appropriate representation based on the specific requirements of your problem to optimize space and time complexity.
  3. Avoid redundant visits: BFS may visit some vertices multiple times if not properly managed. To optimize the algorithm, mark visited vertices to avoid redundant visits and prevent infinite loops.
  4. Handle disconnected graphs: BFS assumes that the graph is connected, meaning that there is a path between any pair of vertices. If the graph is disconnected, meaning that there are isolated vertices or disconnected components, you may need to modify the algorithm to handle such cases.


Breadth-First Search is a powerful and widely used graph traversal algorithm that can be applied to various real-world problems. By understanding its key concepts, algorithmic details, time and space complexity, and practical implementation tips, you can leverage BFS to explore and search for vertices in a graph efficiently.