24-两两交换链表中的节点

https://leetcode-cn.com/problems/swap-nodes-in-pairs/solution/shou-hua-tu-jie-24-liang-liang-jiao-huan-lian-biao/

var swapPairs = function(head) {
  const dummy = new ListNode(null); // 设置虚拟头节点
  dummy.next = head; // 虚拟头节点连上节点
  let pre = dummy;
  while (head && head.next) {
      const temp = head.next;
      head.next = temp.next;
      temp.next = head;
      pre.next = temp;
      pre = head;
      head = head.next
  }
  return dummy.next
};