leetcode-24-swap-nodes-in-pairs.swift

/**

Copyright (c) 2020 David Seek, LLC

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OR OTHER DEALINGS IN THE SOFTWARE.





https://leetcode.com/problems/swap-nodes-in-pairs/


Given a linked list, swap every two adjacent nodes and return its head.

You may not modify the values in the list's nodes, only nodes itself may be changed.

*/

/**
Big O Annotation
Time complexity O(n) where n is the amount of nodes in the list.
Space complexity O(1) we're not using any extra space.
*/
func swapPairs(_ head: ListNode?) -> ListNode? {
        
    // Return is there is no next node
    if head?.next == nil { 
        return head
    }
    
    /**
    Get a reference to the current next node
    In this example: [1,2,3,4],
    we have now [2,3,4]
    */
    let next = head?.next

    /**
    Now we're computing [1]'s next,
    with parameter next?.next what is right now [3,4]
    That will give us [1,3,4]
    and that iteration called again
    will give us [1,4,3]
    */
    head?.next = swapPairs(next?.next)

    // Set [2]'s next to [1,4,3]
    next?.next = head

    // And return
    return next
}