Fix LinkedList demo code in academy.realm.io(https://academy.realm.io/posts/try-swift-soroush-khanlou-sequence-collection/)

LinkedListIterator

/*:
* [How to make custom iterator](https://academy.realm.io/posts/try-swift-soroush-khanlou-sequence-collection/)
* [Implementation of a LinkedList using enum](https://github.com/apple/swift/blob/master/stdlib/public/core/Sequence.swift)
 */
import Foundation

indirect enum LinkedListNode<T> {
    case value(element: T, next: LinkedListNode<T>)
    case end
}

//extension LinkedListNode: Sequence {
//    func makeIterator() -> LinkedListIterator<T> {
//        return LinkedListIterator(current: self)
//    }
//}

// You can use a default implementation for makeIterator()
struct LinkedListIterator<T>: Sequence, IteratorProtocol {

    var current: LinkedListNode<T>

    mutating func next() -> T? {
        switch current {
        case let .value(element, next):
            current = next
            return element
        case .end:
            return nil
        }
    }
}

// create LinkedListNode
let second = LinkedListNode.value(element: "C", next: LinkedListNode.end)
let first = LinkedListNode.value(element: "B", next: second)
let head = LinkedListNode.value(element: "A", next: first)

// use `var` instead of `let`.
// because iterator.next() is a mutating function.
var iterator = LinkedListIterator<String>(current: head)
while let value = iterator.next() {
    print(value)
}