1. Queue
BFS explores nodes in first-in-first-out order.
const queue = [start]Typorithm · medium
Breadth-First Search
▷ Interactive algorithm labbreadth-first-searchsearchgraphqueueshortest-path-unweighted
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Full lesson · 8 learning steps Open any exercise for focused practice
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Step 1 of 8 · intro card
Learn Breadth-First Search
Search Algorithms
Breadth-First Search explores a graph level by level using a queue.
BFS starts from a node and explores all nearby nodes before going deeper.
A queue preserves discovery order.
A visited set prevents revisiting nodes in cycles.
For unweighted graphs, BFS finds the fewest-edge path to a target.
Preparing algorithm lab…
01
Use a queue.
02
Mark nodes visited when discovered.
03
Explore neighbors level by level.
04
Works on adjacency lists.
05
Time is O(V + E).
function breadthFirstSearch(graph, start, target) {
const queue = [start];
const visited = new Set([start]);
while (queue.length > 0) {
const node = queue.shift();
if (node === target) return true;
for (const neighbor of graph[node] ?? []) {
if (!visited.has(neighbor)) {
visited.add(neighbor);
queue.push(neighbor);
}
}
}
return false;
}Read the implementation
Connect each part of the code to the concept before you type.
2. Visited set
visited prevents cycles from repeating forever.
const visited = new Set([start])3. Queue loop
Keep exploring while there are discovered nodes waiting.
while (queue.length > 0)4. Dequeue
Take the oldest discovered node.
const node = queue.shift()5. Target check
Return true when the current node is the target.
if (node === target) return true6. Neighbors
Read neighbors from the adjacency list.
for (const neighbor of graph[node] ?? [])7. Enqueue
Newly discovered neighbors wait behind older nodes.
queue.push(neighbor)A complete learning loop
01
Understand
Read the concept map and follow the code breakdown.
02
Reconstruct
Type the implementation, repair mistakes and work through every recall exercise.
03
Verify
Use the final check without hints; revisit weak concepts in short practice sessions.