Queue / BFS

Level-by-level traversal for shortest paths and layer-based problems.

10 problems 0 Easy 9 Medium 1 Hard

Overview

Breadth-First Search uses a queue to explore nodes level by level, guaranteeing the shortest path in unweighted graphs. It is the foundation for many graph and tree problems.

When to Use

Shortest path in unweighted graph
Level-order tree traversal
Rotting oranges / island flooding
Word ladder
Multi-source BFS

Common Mistakes

Not marking nodes as visited before enqueueing (causes duplicates)
Using BFS when DFS is sufficient and simpler
Forgetting to handle disconnected components

Complexity

Time

O(V + E) for graphs, O(n) for trees

Space

O(V) for the queue

function bfs(graph, start) {
  const visited = new Set([start]);
  const queue = [start];

  while (queue.length) {
    const node = queue.shift();
    // process node

    for (const neighbor of graph[node]) {
      if (!visited.has(neighbor)) {
        visited.add(neighbor);
        queue.push(neighbor);
      }
    }
  }
}

Problems (10)

Hide completed

Problem	Difficulty	Companies	Freq	Link
Number of Islands	Medium	AmazonGoogle +2	High
Rotting Oranges	Medium	AmazonGoogle	High
Binary Tree Level Order Traversal	Medium	AmazonFacebook +1	High
Word Ladder	Hard	AmazonFacebook +2	Medium
01 Matrix	Medium	FacebookAmazon +1	High
Walls and Gates ★	Medium	FacebookAmazon	Medium	★
Shortest Path in Binary Matrix	Medium	AmazonGoogle +1	Medium
Open the Lock	Medium	GoogleAmazon	Medium
Minimum Genetic Mutation	Medium	GoogleAmazon	Medium
Jump Game III	Medium	AmazonGoogle	Medium

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