剑指 Offer II 024. 反转链表

1. while (p != null)

2. 返回 prev 指针

   class Solution {
    public ListNode reverseList(ListNode head) {
        if (head == null) return null;
        ListNode prev = null, p = head;
        while (p != null) {
            ListNode next = p.next;
            p.next = prev;
            prev = p;
            p = next;
        }
        return prev;
    }
   }