Stack.swift

import Foundation

class Stack<T> {
    private typealias Node = StackNode<T>
    private var _top : Node?
    private var _size : UInt = 0
    
    
    //MARK: Public API
    internal func empty() -> Bool {
        return self._top == nil
    }
    
    internal func size() -> UInt {
        return self._size
    }
    
    internal func top() -> T? {
        return self._top?.value
    }
    
    internal func push(node : T?) throws {
        guard let n = node else {
            throw ManipulationErrors.InsertNilElement
        }
        
        if let top = self._top {
            let nextNode = Node()
            nextNode.value = n
            nextNode.next = top
            self._top = nextNode
        } else {
            self._top = Node()
            self._top!.value = n;
        }
        
        self._size += 1;
    }
    
    internal func pop() throws {
        guard let n = self._top else {
            throw ManipulationErrors.PopEmpty
        }
        
        self._top = n.next
        self._size -= 1
    }
}

extension Stack : CustomStringConvertible {
    var description: String {
        var debug = "<\(Stack.self)> ["
        while !self.empty() {
            debug += "\(self.top()!)"
            if self.size() != 1 {
                debug += ","
            }
            do { try self.pop() }
            catch {}
        }
        
        debug += "]"
        return debug
    }
}

StackNode.swift

import Foundation

class StackNode<T> {
    var value : T!
    var next : StackNode?
}