24. 两两交换链表中的节点

/**
 * 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) {
  let ret = new ListNode(0, head),
    temp = ret
  while (temp.next && temp.next.next) {
    let cur = temp.next.next,
      pre = temp.next
    pre.next = cur.next
    cur.next = pre
    temp.next = cur
    temp = pre
  }
  return ret.next
}