SwapNodesInPairs.swift

/**
 * Question Link: https://leetcode.com/problems/swap-nodes-in-pairs/
 * Primary idea: Three Pointers, each time change pointers' directions 
 * Time Complexity: O(n), Space Complexity: O(1)
 *
 * 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
 *     }
 * }
 */

class SwapNodesInPairs {
    func swapPairs(head: ListNode?) -> ListNode? {
        let dummy = ListNode(0)
        dummy.next = head
        
        var prev: ListNode? = dummy
        var current = dummy.next
        
        while current != nil && current!.next != nil {
            let next = current!.next
            let post = current!.next!.next
            
            prev!.next = next
            next!.next = current
            current!.next = post
           
            prev = current
            current = post
        }
        
        return dummy.next
    }
}