# LeetCode Notes: Implement Stack using Queues

## Question

Implement a last-in-first-out (LIFO) stack using only two queues. The implemented stack should support all the functions of a normal stack (`push`, `top`, `pop`, and `empty`).

Implement the `MyStack` class:

• `void push(int x)` Pushes element x to the top of the stack.
• `int pop()` Removes the element on the top of the stack and returns it.
• `int top()` Returns the element on the top of the stack.
• `boolean empty()` Returns `true` if the stack is empty, `false` otherwise.

Notes:

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

Example 1:

Input

["MyStack", "push", "push", "top", "pop", "empty"]

[[], [1], [2], [], [], []]

Output

[null, null, null, 2, 2, false]

Explanation

MyStack myStack = new MyStack();

myStack.push(1);

myStack.push(2);

myStack.top(); // return 2

myStack.pop(); // return 2

myStack.empty(); // return False

Constraints:

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

Follow-up: Can you implement the stack using only one queue?

For questions of the same type, please refer to

232. Use stacks to implement queues

## Solution

Analysis:

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

Code:

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

/**
* Push element x onto stack.
* @param {number} x
* @return {void}
*/
MyStack.prototype.push = function(x) {
this.stack.push(x)
};

/**
* Removes the element on top of the stack and returns that element.
* @return {number}
*/
MyStack.prototype.pop = function() {
return this.stack.pop()
};

/**
* Get the top element.
* @return {number}
*/
MyStack.prototype.top = function() {
return this.stack[this.stack.length - 1]
};

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

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