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Breadth-First Search: Layered Graph Traversal

Explore network graph topologies tier-by-tier with a FIFO queue to guarantee shortest path discovery in unweighted graphs.

Updated Sep 4, 20266 min readIntermediateData Structures & Algorithms

INTERACTIVE ALGORITHM WORKBENCH

Breadth-First Search (BFS) Visualizer

MediumGraph & QueueTime O(V + E)Space O(V)
Graph Network Topology
ABCDE
BFS Execution State
FIFO Queue (Front → Back):
A ◀ Front
Visited Sequence:
A
Active Node—
Probed Neighbor—
Queue Size1
BFS StatusStarting
solution.py
1def bfs(graph, start):
▶2    visited = {start}
3    queue = [start]
4    order = []
5    while queue:
6        node = queue.pop(0)
7        order.append(node)
8        for neighbor in graph[node]:
9            if neighbor not in visited:
10                visited.add(neighbor)
11                queue.append(neighbor)
12    return order
Step 1 of 21
WHY THIS STEP WORKS

Breadth-First Search discovers vertices layer by layer using a First-In, First-Out (FIFO) queue to ensure shortest hop distances.

  1. 1Mark start node "A" as visited
  2. 2Enqueue "A"
  3. 3Queue: [A]