Binary Tree Level Order Traversal
Return node values grouped by depth from left to right.
Why does this pattern fit?
Restate the exact job
Return node values grouped by depth from left to right.
A queue boundary separates the current level from the next.
O(n) time · O(w) space
Using the changing queue length mixes levels.
How to solve Binary Tree Level Order Traversal
The goal is to solve this problem from the pattern, not to memorize a finished answer. Use this as a check after your own attempt.
What the question asks
Return node values grouped by depth from left to right.
Why Trees & recursive traversal fits
A queue boundary separates the current level from the next.
State to maintain
A queue and the level size captured before processing.
Transition
Remove exactly levelSize nodes, enqueue children, and append their values as one level.
Time and space
O(n) time · O(w) space
Counterexample to the tempting mistake
Using the changing queue length mixes levels.
Prove it again tomorrow
Close this page. Rebuild the state and transition from memory, write a test that exposes the mistake above, then solve a fresh input without looking back. A same-day reread is practice, not proof of retention.