reverseLinkedList.swift

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     public var val: Int
 *     public var next: ListNode?
 *     public init(_ val: Int) {
 *         self.val = val
 *         self.next = nil
 *     }
 * }
 */

// Iterative solution
    func reverseList(_ head: ListNode?) -> ListNode? {                
        var head = head
        var newHead: ListNode? = nil
        while head != nil{
            var next = head?.next
            head?.next = newHead
            newHead = head
            head = next
        }
        return newHead
    }

// Recursive solution
func reverseList(_ head: ListNode?) -> ListNode? {                
        return helper(head, nil)
    }
    
    func helper (_ head: ListNode?, _ newHead: ListNode?) -> ListNode?{
        if head == nil{
            return newHead
        }
        let next = head?.next
        head?.next = newHead
        return helper(next, head)
    }