LeetCode Card 259 — Tree DFS / Implement
Pattern signal
This pattern usually appears when the prompt involves recursive subtree questions about depth, identity, path sums, construction, or transformation. Invert Binary Tree, Maximum Depth, Same Tree, Subtree of Another Tree, and Binary Tree Maximum Path Sum belong here.
What Implement means here
In BASIC, the Implement step is where you write the code in the same order as the chosen skeleton so the logic stays stable. For Tree DFS, 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 transcribe the plan instead of improvising. 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
The miss is failing to define the recursion contract: what each call returns and what side effects it performs. 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 DFS, what is the simplest way to write the code in the same order as the chosen skeleton so the logic stays stable?”