How it works?

BFS keeps a visited marker per vertex and a queue of vertices to process.

Step-by-Step Process:

1. Mark the start visited and enqueue it.
2. Dequeue the front vertex and record it in the visit order.
3. For each unvisited neighbor (ascending), mark it visited and enqueue it. Marking on enqueue keeps a vertex from being added twice.
4. Repeat until the queue is empty.

Example on vertices 0..3 with edges [0,1, 0,2, 1,3, 2,3], starting at 0:

• Visit 0, enqueue 1 and 2; visit 1, enqueue 3; visit 2; visit 3.
• Visit order: [0, 1, 2, 3] (compare with DFS: [0, 1, 3, 2]).