// 非递归翻转链表
var reverseList = function(head) {
let pre =null
while(head){
let next = head.next
head.next = pre
pre = head
head = next
}
return pre
}
// 递归翻转链表
var reverseList = function(head) {
if(!head || !head.next){ // 第一个是链表为null的情况,第二个是正常的判断结束条件
return head
}
let reverse = reverseList(head.next)
head.next.next = head
head.next = null
return reverse
}