反转链表

输入: 1->2->3->4->5->NULL
输出: 5->4->3->2->1->NULL

    public ListNode reverseList(ListNode head) {
        //终止条件
        if(head == null || head.next == null){
            return head;
        }

        ListNode p = reverseList(head.next);
        head.next.next = head;
        head.next = null;

        return p;
    }

    public ListNode reverseList(ListNode head) {
        return reverseList(null,head);
    }
    public ListNode reverseList(ListNode lastNode, ListNode curr) {
        if(curr == null){
            return lastNode;
        }
        ListNode next = curr.next;
        curr.next = lastNode;
        return reverseList(curr, next);        
    }

    public ListNode reverseList(ListNode head) {

        ListNode pre = null;
        ListNode curr = head;
        ListNode temp;

        while (curr != null) {
            temp = curr.next;
            curr.next = pre;
            pre = curr;
            curr = temp;
        }

        return pre;
    }