Queue.swift

import Foundation

class Queue<T> {
    private typealias Node = QueueNode<T>
    private var _top : Node?
    private var _last : 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 front() -> T? {
        return self._top?.value
    }
    
    internal func back() -> T? {
        return self._last?.value
    }
    
    internal func push(node : T?) throws {
        guard let n = node else {
            throw ManipulationErrors.InsertNilElement
        }
        
        if let _ = self._top {
            let newNode = Node()
            newNode.value = n
            self._last?.next = newNode
            self._last = newNode
        } else {
            self._top = Node()
            self._top?.value = n
            self._last = self._top
        }
        
        self._size += 1
    }
    
    internal func pop() throws {
        guard let _ = self._top else {
            throw ManipulationErrors.PopEmpty
        }
        
        self._top = self._top?.next
        self._size -= 1
    }
}

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

QueueNode.swift

import Foundation

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