24.反转链表

public ListNode reverseList(ListNode head) {
        
        if(head == null )
             return head;

        ListNode pre = null;
        ListNode cur = head;
        while(cur != null) {
            ListNode  next = cur.next;
            cur.next = pre;
            pre=cur;
            cur=next;
            
        }
        return pre;

        
    }