Exact Change — Solution & Editorial

Back to the Problem

Track your 5s and 10s. For a 20, prefer giving 10+5 over 5+5+5 — 5s are the universal change and must be hoarded.

Complexity: O(N)

Watch out: Breaking a 20 with three 5s while a 10 is available loses on later customers — greed must protect the 5s.