Queue.swift

//
//  Queue.swift
//  SwiftStructures
//
//  Created by Wayne Bishop on 7/11/14.
//  Copyright (c) 2014 Arbutus Software Inc. All rights reserved.
//

import Foundation

class Queue<T> {
        
    private var top: Node<T>?
    private var counter: Int = 0
    
    init() {
      top = Node<T>()
    }
    
    
    //the number of items
    var count: Int {
        return counter
    }


    //MARK: Supporting Functions
    
    
    
    //retrieve the top most item
    func peek() -> T? {
        return top?.key
    }
    
    
    
    //check for the presence of a value
    func isEmpty() -> Bool {
        
        guard top?.key != nil else {
            return true
        }
        return false
    }


    
    //MARK: Queuing Functions

    
    //enqueue the specified object
    func enQueue(_ key: T) {
        
        
        //trivial case
        guard top?.key != nil else {
            top?.key = key
            counter += 1
            return
        }
        
        let childToUse = Node<T>()
        var current = top
    
        
        //cycle through the list
        while current?.next != nil {
            current = current?.next
        }
        
        
        //append new item
        childToUse.key = key
        current?.next = childToUse
        counter += 1
    }
    

    
    //retrieve items from the top level - O(1) time
   func deQueue() -> T? {
    
    
        //determine key instance
        guard top?.key != nil else {
            return nil
        }
    
        
        //retrieve and queue the next item
        let queueItem: T? = top?.key
    
    
        //use optional binding 
        if let nextItem = top?.next {
          top = nextItem
        }
        else {
            top = Node<T>()
        }
    
        counter -= 1
    
        return queueItem
        
    }
    
    
    
} //end class