public void clear() {
head = new DoubleLinkedNode(0, 0);
tail = new DoubleLinkedNode(0, 0);
head.next = tail;
tail.pre = head;
map = new HashMap<Integer, DoubleLinkedNode>();
}
public void add(int key, int val) {
DoubleLinkedNode node = new DoubleLinkedNode(key, val);
node.next = head.next;
head.next.pre = node;
head.next = node;
node.pre = head;
map.put(key, node);
}