0024-swap-nodes-in-pairs.js

/**
 * 24. Swap Nodes in Pairs
 * https://leetcode.com/problems/swap-nodes-in-pairs/
 * Difficulty: Medium
 *
 * Given a linked list, swap every two adjacent nodes and return its head. You must solve the
 * problem without modifying the values in the list's nodes (i.e., only nodes themselves may
 * be changed.)
 */

/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} head
 * @return {ListNode}
 */
var swapPairs = function(head) {
  if (!head || !head.next) return head;

  const result = head.next;
  head.next = result.next;
  result.next = head;
  head.next = swapPairs(head.next);

  return result;
};

// var swapPairs = function(head) {
//   let dummy = new LinkedListNode()
//   dummy.next = head

//   current = dummy

//   while (current.next && current.next.next) {
//       first = current.next
//       second = current.next.next

//       first.next = second.next
//       second.next = first
//       current.next = second

//       current = first
//   }

//   return dummy.next
// };