mutable struct Queue
    front::LinkedList
    rear::LinkedList
    size::Int
end

function push!(q::Queue, elm)
    if q.rear == nothing
        q.front = q.rear = ListNode(elm, nothing)
        q.size = 1
    else
        node = ListNode(elm, nothing)
        q.rear.next = node
        q.rear = node
        q.size += 1
    end
end

function pop!(q::Queue)
    if q.front == nothing
        error("empty queue")
    else
        x = q.front.data
        q.front = q.front.next
        if q.front == nothing
            q.rear = nothing
        end
        q.size -= 1
        return x
    end
end

function isempty(q::Queue)
    return q.size == 0
end

q = Queue(nothing, nothing, 0)
push!(q, 1)
push!(q, 2)
push!(q, 3)
println(pop!(q))
println(q)