leetcode-21-merge-two-sorted-lists.swift

/**

Copyright (c) 2020 David Seek, LLC

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https://leetcode.com/problemset/all/


Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.

Example:

Input: 1->2->4, 1->3->4
Output: 1->1->2->3->4->4

*/

/**
Big O Annotation
Time complexity O(n) where n is the combined amount of nodes in l1 + l2.
Space complexity O(n) where n is the combined amount of nodes in l1 + l2.
*/
func mergeTwoLists(_ l1: ListNode?, _ l2: ListNode?) -> ListNode? {
	
    // Check if l1 exists
    guard let l1 = l1 else {
        return l2
    }
    
    // Check if l2 exists
    guard let l2 = l2 else {
        return l1
    }
    
    // Get a current node reference and a head to lose
    var current: ListNode? = ListNode(-1)

    // Keep a reference of the head
    var head: ListNode = current!
    
    // Get the current list references
    var currentL1: ListNode? = l1
    var currentL2: ListNode? = l2
    
    // While both lists still have work to do
    while currentL1 != nil && currentL2 != nil {
        
        // Check if l1 is smaller or l2
        if currentL1!.val <= currentL2!.val {

            // Append the smaller element
            current!.next = currentL1

            // And update the reference
            currentL1 = currentL1!.next
        } else {

            // Same as above
            current!.next = currentL2
            currentL2 = currentL2!.next
        }
        
        // Move current along
        current = current?.next
    }
    
    // And append missing elements
    if let list = currentL1 {
        current?.next = list
    }
    
    // Sane as above
    if let list = currentL2 {
        current?.next = list
    }
    
    // Return head.next to lose the -1 node
    return head.next
}