# LeetCode Notes: Implement Queue using Stacks

## Question

Implement a first in first out (FIFO) queue using only two stacks. The implemented queue should support all the functions of a normal queue (`push`, `peek`, `pop`, and `empty`).

Implement the `MyQueue` class:

• `void push(int x)` Pushes element x to the back of the queue.
• `int pop()` Removes the element from the front of the queue and returns it.
• `int peek()` Returns the element at the front of the queue.
• `boolean empty()` Returns `true` if the queue is empty, `false` otherwise.

Notes:

• You must use only standard operations of a stack, which means only `push to top`, `peek/pop from top`, `size`, and `is empty` operations are valid.
• Depending on your language, the stack may not be supported natively. You may simulate a stack using a list or deque (double-ended queue) as long as you use only a stack's standard operations.

Example 1:

Input

["MyQueue", "push", "push", "peek", "pop", "empty"]

[[], , , [], [], []]

Output

[null, null, null, 1, 1, false]

Explanation

MyQueue myQueue = new MyQueue();

myQueue.push(1); // queue is: 

myQueue.push(2); // queue is: [1, 2] (leftmost is front of the queue)

myQueue.peek(); // return 1

myQueue.pop(); // return 1, queue is 

myQueue.empty(); // return false

Constraints:

• `1 <= x <= 9`
• At most `100` calls will be made to `push`, `pop`, `peek`, and `empty`.
• All the calls to `pop` and `peek` are valid.

Follow-up: Can you implement the queue such that each operation is amortized `O(1)` time complexity? In other words, performing `n` operations will take overall `O(n)` time even if one of those operations may take longer.

For questions of the same type, please refer to

225. Implement Stack using Queues

## Solution

Analysis:

Take advantage of the characteristics of the js array, where the top of the stack is at the head of the array.

Code:

``````/**
* Initialize your data structure here.
*/
var MyQueue = function() {
this.queue = []
};

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

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

/**
* Get the front element.
* @return {number}
*/
MyQueue.prototype.peek = function() {
return this.queue
};

/**
* Returns whether the queue is empty.
* @return {boolean}
*/
MyQueue.prototype.empty = function() {
return this.queue.length === 0
};

/**
* Your MyQueue object will be instantiated and called as such:
* var obj = new MyQueue()
* obj.push(x)
* var param_2 = obj.pop()
* var param_3 = obj.peek()
* var param_4 = obj.empty()
*/
``````