leetcode-206-reverse-linked-list.swift

/**

Copyright (c) 2020 David Seek, LLC

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https://leetcode.com/problems/reverse-linked-list/


Reverse a singly linked list.

Example:

Input: 1->2->3->4->5->NULL
Output: 5->4->3->2->1->NULL
Follow up:

A linked list can be reversed either iteratively or recursively. Could you implement both?

*/

/**
Big O Annotation
Time complexity O(n) where n is the amount of node in head.
Space complexity O(n) where n is the amount of node in head.
*/
func reverseList(_ head: ListNode?) -> ListNode? {
        
    // Withouta head nothing to reverse
    guard let originalHead = head else {
        return nil
    }
    
    var values: [Int] = []
    
    // Create a new dummy root
    var root: ListNode? = ListNode(-1)

    // And store the reference to the head
    var head: ListNode? = root
    
    // Traverse the original list
    originalHead.traverse { values.append($0) }
    
    // And while the list of values is not empty...
    while !values.isEmpty {
        
        // ... get the last element of the array
        guard let value = values.popLast() else {
            break
        }
        
        // Add it as the next element of the current node
        root?.next = ListNode(value)

        // And set the current node as the next "current"
        root = root?.next
    }
    
    /**
    Return the next of head to 
    lose the -1 temp node
    */
    return head?.next
}

extension ListNode {

    // Function to traverse the list
    func traverse(visit: (Int) -> Void) {
        visit(val)
        next?.traverse(visit: visit)
    }
}