输入一个链表，反转链表后，输出新链表的表头。

public class Solution {
    public ListNode ReverseList(ListNode head) {
        if(head==null) return null;
        ListNode pre = head;
        ListNode next = head.next;
        pre.next=null;// !!
        while(next!=null){
            ListNode temp = next.next;
            next.next= pre;
            pre= next;
            next = temp;
        }
        return pre;
    }
}