单链表的翻转

public ListNode ReverseList(ListNode head) {
if(head == null){
return head;
}
ListNode p1 = head;
ListNode p2 = head.next;
ListNode p3;
while(p2!=null){
p3 = p2.next;
p2.next = p1;
p1 = p2;
p2 = p3;
}
head.next = null;
head = p1;
return head;
}