// 输入一个链表,反转链表后,输出新链表的表头。
/*function ListNode(x){
this.val = x;
this.next = null;
}*/
function ReverseList (head) {
if (head === null) {
return null
}
let pre = null
let next = null
while (head !== null) {
next = head.next
head.next = pre
pre = head
head = next
}
return pre
}