1. Mark on enqueue
Mark a node visited the moment it enters the queue, not when it leaves.
const visited = new Set([start])Typorithm · medium
Graph BFS
▷ Interactive algorithm labgraph-bfsgraph-algorithmsqueuevisited-setlevels
Saved on this device
Full lesson · 8 learning steps Open any exercise for focused practice
10%
Step 1 of 8 · intro card
Learn Graph BFS
Graph Algorithms
Explore a graph level by level with a FIFO queue, marking nodes visited as they are enqueued.
BFS visits all neighbors of a node before going deeper, expanding outward in rings.
A visited set prevents revisiting and, marked on enqueue, stops a node entering the queue twice.
In an unweighted graph, the first time BFS reaches a node is along a shortest path.
Preparing algorithm lab…
01
Queue gives level order.
02
Mark visited on enqueue.
03
First reach is shortest (unweighted).
function bfs(graph, start) {
const visited = new Set([start]);
const queue = [start];
const order = [];
while (queue.length > 0) {
const node = queue.shift();
order.push(node);
for (const next of graph[node]) {
if (!visited.has(next)) {
visited.add(next);
queue.push(next);
}
}
}
return order;
}Read the implementation
Connect each part of the code to the concept before you type.
2. FIFO frontier
A queue processes nodes oldest-first, giving level-by-level order.
const queue = [start]3. Dequeue
Take the front node to expand its neighbors.
const node = queue.shift()4. Skip visited
Never enqueue a node that is already seen.
if (!visited.has(next))5. Enqueue neighbor
Add unseen neighbors to the back of the queue.
queue.push(next)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.