反转单向链表

206. Reverse Linked List

思路：迭代，将链表分为头结点head和后续链表reslist。假设reslist已完成反转，relist的头结点为rehead: rehead->head；head->null;

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