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

//Time complexity: O(n)

func swapPairs(_ head: ListNode?) -> ListNode? {
    guard let head = head else { return nil }
    if head.next == nil { return head }

    var newHead, ptr1, ptr2, ptr3, ptr4: ListNode?
    ptr1 = nil
    ptr2 = head
    ptr3 = head.next
    newHead = ptr3
    ptr4 = head.next?.next

    while ptr2 != nil && ptr3 != nil {
        ptr3?.next = ptr2
        ptr2?.next = ptr4
        ptr1?.next = ptr3
        
        ptr1 = ptr2
        ptr2 = ptr4
        ptr3 = ptr4?.next
        ptr4 = ptr4?.next?.next
    }

    return newHead
}