232. Implement Queue using Stacks

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.

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.

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.

# @lc code=start
using LeetCode
using DataStructures

struct MyQueue
    in_stack::Vector{Int}
    out_stack::Vector{Int}
    MyQueue() = new(Int[], Int[])
end


# Push element x to the back of queue.
myqueue_push!(queue::MyQueue, x::Int) = push!(queue.in_stack, x)

# Removes the element from in front of queue and returns that element.
function myqueue_pop!(queue::MyQueue)
    myqueue_move!(queue)

    return pop!(queue.out_stack)
end

# Get the front element.
function myqueue_peek(queue::MyQueue)
    myqueue_move!(queue)
    return queue.out_stack[end]
end

# Returns whether the queue is empty.
myqueue_isempty(queue::MyQueue) = isempty(queue.in_stack) && isempty(queue.out_stack)

function myqueue_move!(queue::MyQueue)
    if isempty(queue.out_stack)
        while !isempty(queue.in_stack)
            push!(queue.out_stack, pop!(queue.in_stack))
        end
    end
end
# @lc code=end

myqueue_move! (generic function with 1 method)

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