Solution to LeetCode problem, 225 Implement Stack using Queues. https://leetcode.com/problems/implement-stack-using-queues/

stack_using_queues.swift

class ListNode {
    var next: ListNode?
    var key: Int = 0
}
/*
 push to back, peek/pop from front, size and is empty
 */
class LinkedList {
    private var head: ListNode?
    private var tail: ListNode?
    private var count: Int = 0
    
    func push(_ key: Int) {
        let newNode = ListNode()
        newNode.key = key
        
        if head == nil {
            head = newNode
            tail = newNode
        }
        else {
            tail?.next = newNode
            tail = tail?.next
        }
        
        count += 1
    }
    
    func peak() -> Int {
        return head?.key ?? 0
    }
    
    func popFront() -> Int {
        let frontNode = head
        head = head?.next
        
        if head == nil {
            tail = nil
        }
        
        count -= 1
        
        return frontNode?.key ?? 0
    }
    
    func size() -> Int {
        return count
    }
    
    func isEmpty() -> Bool {
        return size() == 0
    }
}


class MyStack {

    private var frontQueue = LinkedList()
    private var backQueue = LinkedList()
    
    init() {
        
    }
    
    func push(_ x: Int) {
        if frontQueue.size() > 0 {
            backQueue.push(x)
            
            while frontQueue.size() > 0 {
                let val = frontQueue.popFront()
                backQueue.push(val)
            }
            
            swapQueues()
        }
        else {
            frontQueue.push(x)
        }
    }
    
    func pop() -> Int {
        let val = frontQueue.popFront()
        return val
    }
    
    func top() -> Int {
        return frontQueue.peak()
    }
    
    func empty() -> Bool {
        return frontQueue.isEmpty()
    }
    
    private func swapQueues() {
        let temp = frontQueue
        frontQueue = backQueue
        backQueue = temp
    }
}

/**
 * Your MyStack object will be instantiated and called as such:
 * let obj = MyStack()
 * obj.push(x)
 * let ret_2: Int = obj.pop()
 * let ret_3: Int = obj.top()
 * let ret_4: Bool = obj.empty()
 */