Stack implementation in Swift 3.1

Stack.swift

import UIKit

// Stack implementation in Swift 3.1

class Node {
    let value: Int
    var nextNode: Node?
    
    init(value: Int) {
        self.value = value
    }
}

class Stack {
    
    var top: Node?
    
    func push(_ value: Int) {
        let oldTop = top
        top = Node(value: value)
        top?.nextNode = oldTop
    }
    
    func pop() -> Int? {
        let currentTop = top
        top = top?.nextNode
        return currentTop?.value
    }
}

let stack = Stack()
stack.push(12)
stack.push(22)
stack.push(32)

stack.pop() // 32
stack.pop() // 22
stack.pop() // 12
stack.pop() // nil