翻转链表

public ListNode reverse(ListNode head) {
ListNode head1 = head, p = null;
if (head == null || head.next == null) return head1;
head = head.next;
head1.next = null;
while (head != null) {
p = head;
head = head.next;
p.next = head1;
head1 = p;
}
return head1;
}