Depth First Search

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November 1, 2022

Coding & Algorithm Interview 


Depth First Search (DFS) is a classic graph traversal algorithm that has widespread applications in various areas of computer science, such as graph theory, network analysis, and artificial intelligence. It is a fundamental algorithm that explores the depths of a graph, uncovering its connected components, finding paths, and detecting cycles. In this tech blog, we will provide a detailed overview of the DFS algorithm, its implementation, time and space complexities, and practical use cases.

What is Depth First Search (DFS)?

DFS is a graph traversal algorithm that explores a graph by starting from a source vertex and visiting its adjacent vertices until all reachable vertices are visited, and there are no more unexplored vertices left. The algorithm follows a depth-first approach, meaning that it explores as far as possible along each branch before backtracking. DFS uses a stack data structure to keep track of the vertices to visit, and it employs the concept of recursion to implement the backtracking mechanism.

DFS Algorithm

The DFS algorithm can be summarized in the following steps:

  1. Choose a source vertex to start the traversal.
  2. Mark the source vertex as visited.
  3. Visit each unvisited adjacent vertex of the current vertex recursively until there are no more unvisited vertices.
  4. If there are no unvisited vertices, backtrack to the previous vertex and continue the traversal until all vertices are visited.

DFS can be implemented using either an iterative or a recursive approach. Let’s look at a Python implementation of the recursive version:

def dfs(graph, start, visited):
    # Mark the current vertex as visited
    visited[start] = True
    print(start, end=' ')

    # Recur for all the vertices adjacent to this vertex
    for v in graph[start]:
        if not visited[v]:
            dfs(graph, v, visited)

Time Complexity: The time complexity of DFS is O(V + E), where V is the number of vertices and E is the number of edges in the graph. In the worst case, DFS may visit all the vertices and edges of the graph.

Space Complexity: The space complexity of DFS is O(V), as it requires a stack to store the vertices to be visited. In the worst case, the stack may contain all the vertices of the graph.

Applications of DFS

DFS has a wide range of applications in computer science, including but not limited to:

  1. Graph traversal: DFS can be used to traverse a graph and visit all its vertices or find a path between two vertices.
  2. Connected components: DFS can identify all the connected components in an undirected graph.
  3. Cycle detection: DFS can detect cycles in a graph, which is useful in identifying potential deadlocks or loops in a system.
  4. Topological sorting: DFS can be used to perform topological sorting of vertices in a directed acyclic graph (DAG), which has applications in scheduling tasks with dependencies.
  5. Maze solving: DFS can be used to solve mazes by exploring all possible paths from the start to the end point.


DFS is a powerful graph traversal algorithm that provides valuable insights into the structure and connectivity of a graph. It can be implemented using either an iterative or a recursive approach and has widespread applications in various areas of computer science. Understanding the DFS algorithm and its implementation can be beneficial for solving a wide range of problems that involve graph analysis, path finding, and cycle detection. I hope this tech blog has provided you with a comprehensive overview of DFS and its practical applications. Happy exploring the depths with DFS in your future coding endeavors!