Leetcode 232 - Implement queue using stacks

Note:

  • DO NOT use methods like slice, splice, or just assigning values based on index. That’s not using stack!
  • Use pop because what we get will be the last element added.
  • Use 2 stacks (arrays) with methods like pop(), push().
  • StackIn is the stack to push to when you want to insert.
  • StackOut is the stack to contain reversed elements from stackIn. In that way, when we do pop on stackOut, what we get will be the first element of stackIn.
  • Learn to reuse methods that we’ve implemented.

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:

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Input
["MyQueue", "push", "push", "peek", "pop", "empty"]
[[], [1], [2], [], [], []]
Output
[null, null, null, 1, 1, false]

Explanation
MyQueue myQueue = new MyQueue();
myQueue.push(1); // queue is: [1]
myQueue.push(2); // queue is: [1, 2] (leftmost is front of the queue)
myQueue.peek(); // return 1
myQueue.pop(); // return 1, queue is [2]
myQueue.empty(); // return false

Code:

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var MyQueue = function() {
this.stackIn = [];
this.stackOut = [];
};

/**
* @param {number} x
* @return {void}
*/
MyQueue.prototype.push = function(x) {
this.stackIn.push(x);
};

/**
* @return {number}
*/
MyQueue.prototype.pop = function() {
/* This length check is essential! */
if (!this.stackOut.length) {
while(this.stackIn.length) {
this.stackOut.push(this.stackIn.pop());
}
}
return this.stackOut.pop();
};

/**
* @return {number}
*/
MyQueue.prototype.peek = function() {
/* Resue is important */
const res = this.pop();
this.stackOut.push(res);
return res;
};

/**
* @return {boolean}
*/
MyQueue.prototype.empty = function() {
return !this.stackOut.length && !this.stackIn.length;
};

/**
* 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()
*/