232. Implement Queue using Stacks

原题：232. Implement Queue using Stacks
类似的题有：225. Implement Stack using Queues

栈是FILO，队列是FIFO，想用栈实现队列，用的是 “负负得正” 的思想。

Source Code

class Stack {
    constructor() {
        this.stack = [];
    }
    push(x) {
        this.stack.push(x);
    }
    pop() {
        return this.stack.pop();
    }
    empty() {
        return !this.stack.length;
    }
}

var MyQueue = function() {
    this.input = new Stack();
    this.output = new Stack();
};

/**
 * Push element x to the back of queue. 
 * @param {number} x
 * @return {void}
 */
MyQueue.prototype.push = function(x) {
    this.input.push(x);
};

/**
 * Removes the element from in front of queue and returns that element.
 * @return {number}
 */
MyQueue.prototype.pop = function() {
    this._adjust();
    return this.output.pop();
};

/**
 * Get the front element.
 * @return {number}
 */
MyQueue.prototype.peek = function() {
    this._adjust();
    const peekEl = this.output.pop();
    this.output.push(peekEl);

    return peekEl;
};

MyQueue.prototype._adjust = function() {
    if(!this.output.empty()) {
        return;
    }

    while(!this.input.empty()) {
        this.output.push(this.input.pop());
    }
};

/**
 * Returns whether the queue is empty.
 * @return {boolean}
 */
MyQueue.prototype.empty = function() {
    return this.input.empty() && this.output.empty();
};

Test Cases

const queue = new MyQueue();
queue.push(1);
queue.push(2);
test("test1", ()=>{
    expect(queue.peek()).toEqual(1);
})

test("test2", ()=>{
    expect(queue.pop()).toEqual(1);
})

test("test3", ()=>{
    expect(queue.empty()).toEqual(false);
})

test("test4", ()=>{
    queue.pop();
    expect(queue.empty()).toEqual(true);
})