Two Kinds of Stamps — Solution & Editorial

Back to the Problem

T must be a multiple of g = gcd(A, B); then check small residues: iterate k = number of A-stamps modulo B/g — equivalently test T − kA divisible by B for k in [0, min(T/A, B/g−1)]. That bound keeps the loop tiny.

Complexity: O(1) or O(log)