implement_queue_using_stacks.rb

# https://leetcode.com/problems/implement-queue-using-stacks/
#
# Implement the following operations of a queue using stacks.
#
#     + push(x) -- Push element x to the back of queue.
#     + pop() -- Removes the element from in front of queue.
#     + peek() -- Get the front element.
#     + empty() -- Return whether the queue is empty.
#
# 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, stack may not be supported natively. You
#       may simulate a stack by using a list or deque (double-ended queue), as
#       long as you use only standard operations of a stack.
#     + You may assume that all operations are valid (for example,no pop or
#       peek operations will be called on an empty queue).


class Queue
  # Initialize your data structure here.
  def initialize
    @in, @out = [], []
  end

  # @param {Integer} x
  # @return {void}
  def push(x)
    @in.push(x)
  end

  # @return {void}
  def pop
    if @out.empty?
      @out.push(@in.pop()) until @in.empty?
    end

    @out.pop; nil
  end

  # @return {Integer}
  def peek
    if @out.empty?
      @out.push(@in.pop()) until @in.empty?
    end

    @out[-1]
  end

  # @return {Boolean}
  def empty
    @in.empty? && @out.empty?
  end
end