Binary Tree Level Order Traversal — Algorithm interview question

Given a binary tree, explain how you would return its level order traversal: the values of nodes at each level grouped together, from left to right.

For example:

Output: [[3], [9, 20], [15, 7]]

How do you keep track of which nodes belong to which level? What data structure is most suitable here?

Explain your approach and analyze the time and space complexity.

How to approach it

Hint 1
BFS (breadth-first search) naturally processes nodes level by level.
Hint 2
Use a queue. At each level, note how many nodes are currently in the queue - that is the number of nodes at this level.
Hint 3
Process exactly that many nodes, adding their children to the queue for the next level. Collect values into a list for the current level.

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