迭代写法

    public ListNode reverseList(ListNode head) {
        if(head==null||head.next==null){
            return head;
        }
        ListNode a=head,b=a.next;
        while(b!=null){
            ListNode c=b.next;
            b.next=a;
            a=b;
            b=c;
        }
        head.next=null;
        return a;
    }

递归写法

    public ListNode reverseList(ListNode head) {
        if(head==null||head.next==null){
            return head;
        }
        ListNode tail=reverseList(head.next);
        head.next.next=head;
        head.next=null;
        return tail;
    }