/*
Swap Nodes in Pairs
https://leetcode.com/problems/swap-nodes-in-pairs/

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

Example:

Given 1->2->3->4, you should return the list as 2->1->4->3.
Note:

Your algorithm should use only constant extra space.
You may not modify the values in the list's nodes, only nodes itself may be changed.
*/

/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */
/**
 * @param {ListNode} head
 * @return {ListNode}
 */
var swapPairs = function (head) {
  if (head === null || head.next === null) return head;
  var previous = null;
  var current = head;
  var following = head.next != null ? head.next.next : null;
  head = head.next;

  while (current !== null && current.next !== null) {
    var next = current.next;
    next.next = current;
    if (previous != null) previous.next = next;
    current.next = following;
    previous = current;
    current = following;
    following =
      current !== null && current.next != null ? current.next.next : null;
  }

  return head;
};

module.exports.swapPairs = swapPairs;