Pattern Recognition Drill
Medium Stacks & Queues
The Problem
Implement a FIFO queue using only two stacks.
What approach would you use?
Think about it before scrolling down.
Push to inbox stack. On dequeue, transfer inbox → outbox (reversal). Amortized O(1) per operation.
Common Trap
Only transfer to outbox when outbox is empty — transferring every time is O(n) per dequeue.