LinkedLists.swift

class Node<T: Equatable>: Equatable {
    var value: T
    var next: Node<T>? = nil
    var prev: Node<T>? = nil
    
    init(value: T) {
        self.value = value
    }
    
    static func == (lhs: Node<T>, rhs: Node<T>) -> Bool {
        return lhs.value == rhs.value
    }
}


class DoublyLinkedList<T: Equatable> {
    var head: Node<T>? = nil
    var tail: Node<T>? = nil
    var size: Int = 0
    
    func append(value: T) {
        // 1
        let newNode = Node(value: value)
        
        // 2
        guard self.head != nil else {
            // 3
            self.head = newNode
            self.tail = newNode
            self.size += 1
            return
        }
        
        // 4
        self.tail?.next = newNode
        newNode.prev = self.tail
        self.tail = newNode
        
        // 5
        self.size += 1
    }
}