两两交换链表中的节点
var swapPairs = function(head) {
if (head === null || head.next === null) {
return head;
}
let first = head;
let second = head.next;
let others = head.next.next;
// 先把前两个元素翻转
second.next = first;
first.next = swapPairs(others);
return second;
};