public void add(int x) {
Node t = new Node();
t.data = x;
Node tail = getTail();
if (tail == null) head = t;
else tail.next = t;
}
public static Node reverseList(Node head) {
if (head == null) return head;
Node pre = head;
Node cur = head.next;
Node next;
while (cur != null) {
next = cur.next;
cur.next = pre;
pre = cur;
cur = next;
}
head.next = null;
head = pre;
return head;
}