232. Implement Queue using Stacks

Implement a first-in-first-out queue using only two stacks, supporting push, pop, peek and empty. Input is a LeetCode-style list of operations.

Push onto an inbox stack. When you need the front and the outbox is empty, pour the whole inbox into the outbox, which reverses the order. Each element moves at most once.

1class MyQueue {
2  inbox: number[] = []; outbox: number[] = [];
3  push(x: number): void { this.inbox.push(x); }
4  private shift(): void {
5    if (!this.outbox.length)
6      while (this.inbox.length) this.outbox.push(this.inbox.pop()!);
7  }
8  pop(): number { this.shift(); return this.outbox.pop()!; }
9  peek(): number { this.shift(); return this.outbox.at(-1)!; }
10  empty(): boolean { return !this.inbox.length && !this.outbox.length; }
11}

op = MyQueue()

inbox (top = right)

empty

outbox (top = right = front)

empty

output

null

Create inbox and outbox.

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