35. Reverse Linked List

Reverse a singly linked list.

这道题有Recursive和Iterative两种解法，这里先写Recursive的
需要好好思考一下。

public ListNode reverse(ListNode head) {
        // write your code here
        ListNode prev = null;
        ListNode curr = head;
        while (curr != null) {
            ListNode currnext = curr.next;
            curr.next = prev;
            prev = curr;
            curr = currnext;
        }
        return prev;
    }