反转单向链表

  public ListNode ReverseList(ListNode head) {

    ListNode cur = head;

    ListNode pre = null;

    ListNode next = null;

    while (cur != null) {

        next = cur.next;

        cur.next = pre;

        pre = cur;

        cur = next;

    }

    return pre;

    }

 public ListNode ReverseList(ListNode head) {

    ListNode cur = head;

            ListNode pre = head;

            ListNode next = null;

           //借助栈的数据结构

            Stack stack = new Stack();

            while(cur!=null){

                stack.push(cur.val);

                cur=cur.next;

            }

            while(!stack.isEmpty()){

                pre.val=(int) stack.pop();

                pre=pre.next;

            }

            return head;

    }

public ListNode ReverseList(ListNode head) {

        if (null == head || null == head.next) {  

          return head;  

             }  

        ListNode reversedHead = ReverseList(head.next);  

        head.next.next=head;  

        head.next=null; 

        return reversedHead; 

            }