Stack Implementation

Stack.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
    }
}