Reverse a Linked List

ReverseLinkedList.swift

//  Created by Alex Paul on 9/16/20.
//  Copyright © 2020 Alex Paul. All rights reserved.
//

import Foundation

// Reverse a singly linked list.

class Node {
  var value: Int
  var next: Node?
  //var previous: Node?
  
  init(_ value: Int) {
    self.value = value
  }
}

func printNodeValues(_ node: Node?) {
  var node = node
  while let currentNode = node {
    print(currentNode.value)
    node = currentNode.next
  }
}


// 3 -> 1 -> 8 -> nil

let nodeOne = Node(3)
let nodeTwo = Node(1)
let nodeThree = Node(8)

nodeOne.next = nodeTwo
nodeTwo.next = nodeThree

printNodeValues(nodeOne) // 3, 1, 8


// in this problem there is pointer manipulation being done
// what does that mean?
// current node point 3 -> 1
// 3 -> 1 -> 8 -> nil
// 8 -> 1 -> 3 -> nil
func reverseList(_ node: Node?) -> Node? {
  var node = node // now we can mutate node
  
  var previousNode: Node?  // nil - this will be the reverse list returned
  var nextNode: Node? // temp Node
  
  while let currentNode = node {
    nextNode = currentNode.next
    
    // main part of this problem is the following two lines below:
    // this line will be reversing the current next pointer
    // ->
    // <-
    currentNode.next = previousNode
    previousNode = currentNode // nil, 3, 1, 8
    
    // keep traversing the list
    node = nextNode
  }
 
  return previousNode // reverse nodes
}

let modifiedNode = reverseList(nodeOne)

printNodeValues(modifiedNode) // 8, 1, 3