单链表的反转

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

    }