Solution to LeetCode problem, 232 Implement Queue using Stacks. https://leetcode.com/problems/implement-queue-using-stacks/

queue_using_stacks.swift

private class ListNode {
    var next: ListNode?
    var prev: ListNode?
    var key: Int = 0
}

class Stack {
    private var head: ListNode?
    private var tail: ListNode?
    private var count: Int = 0
    
    func push(_ key: Int) {
        let newNode = ListNode()
        newNode.key = key
        
        if tail == nil {
            tail = newNode
            head = newNode
        }
        else {
            newNode.next = head
            head = newNode
        }
        
        count += 1
    }
    
    func pop() -> Int {
        let lastNode = head
        head = lastNode?.next
        
        if head == nil {
            tail = nil
        }
        
        count -= 1
        
        return lastNode?.key ?? 0
    }
    
    func peak() -> Int {
        return head?.key ?? 0
    }
    
    func size() -> Int {
        return count
    }
    
    func isEmpty() -> Bool {
        return size() == 0
    }
}


class MyQueue {
    private var frontStack = Stack()
    private var backStack = Stack()
    private var count = 0
    
    init() {
        
    }
    
    func push(_ x: Int) {        
        frontStack.push(x)
        count += 1
    }
    
    func pop() -> Int {
        if backStack.size() > 0 {
            count -= 1
            return backStack.pop()
        }
        else {
            while frontStack.size() > 0 {
                backStack.push(frontStack.pop())
            }
            
            if backStack.size() > 0 {
                count -= 1
                return backStack.pop()
            }
            
            return 0
        }
    }
    
    func peek() -> Int {
        if backStack.size() > 0 {
            return backStack.peak()
        }
        else {
            while frontStack.size() > 0 {
                backStack.push(frontStack.pop())
            }
            
            if backStack.size() > 0 {
                return backStack.peak()
            }
        }
        
        return 0
    }
    
    func empty() -> Bool {
        return count == 0
    }
    
}

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