LeetCode Card 263 — Tree BFS / Structure
Pattern signal
This pattern usually appears when the prompt involves level-by-level grouping, shortest unweighted path from a root, or nearest-layer reasoning. Binary Tree Level Order Traversal, Zigzag Level Order, Right Side View, and minimum-depth style prompts point here.
What Structure means here
In BASIC, the Structure step is where you state the invariant, helper contract, or control-flow skeleton before coding. For Tree BFS, that matters because the pattern only becomes useful once the candidate is explicit about what is being tracked, reduced, or preserved.
Interview move
A strong move is to make the plan visible before full execution. In this pattern family, say out loud what representation makes the problem easier: the map entry, the pointer invariant, the recursion contract, the queue contents, or the DP state. That keeps the implementation attached to a reason.
Common miss
Candidates forget to separate one level from the next and accidentally flatten the result. BASIC reduces that risk because the stage sequence forces you to earn the implementation instead of jumping straight into it.
BASIC prompt
“Given that this looks like Tree BFS, what is the simplest way to state the invariant, helper contract, or control-flow skeleton before coding?”