LeetCode Card 353 — Math and Combinatorics / Structure
Pattern signal
This pattern usually appears when the prompt involves counting, modular arithmetic, number properties, or formula-driven reduction. Power functions, combination counting, matrix exponentiation variants, and arithmetic pattern questions live 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 Math and Combinatorics, 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 often brute-force because they do not first inspect algebraic structure or constraints. 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 Math and Combinatorics, what is the simplest way to state the invariant, helper contract, or control-flow skeleton before coding?”