0232-implement-queue-using-stacks.rb

# frozen_string_literal: true

# 232. Implement Queue using Stacks
# https://leetcode.com/problems/implement-queue-using-stacks/
# Easy

=begin
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"]
[[], [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


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.
=end

class MyQueue
  def initialize
    @in, @out = [], []
  end

  def push(x)
    @in << x
  end

  def pop
    peek
    @out.pop
  end

  def peek
    @out << @in.pop until @in.empty? if @out.empty?
    @out.last
  end

  def empty
    @in.empty? && @out.empty?
  end
end

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