Microsoft interview #3 — Implement Queue using Stacks

Problem statement
Imagine you’re working on a language which only supports Stack, but you need a Queue functionality to fulfill the requirement. Design a queue that supports standard operations using stacks.

Brief idea (EN)
Use two stacks: in and out.

• enqueue(x): in.push(x)
• dequeue()/front(): if out is empty, move all items from in to out; then pop/peek from out.
• empty(): true if both stacks are empty.
  Amortized O(1) per operation, worst-case O(n) when transferring.

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