想知呢個approach general嚟講係咪optimal？
呢個approach係用red black tree store個Limit
Order係一個linked list同會point to 相對Limit
每個Limit都會store head Oeder同 tail Order
有個hashmap store曬所有Order
一個pointer to smallest Limit
一個pointer to highest Limit
Time complexity：
Add Order: O(logN) for Traversing binary tree, O(1) for adding Order to linked list and Hashmap
Cancel Order: O(1), use hashmap for quick reference
Execute Order: O(1), use smallest or highest limit pointer to get the limit for Buy order or Sell order. Remove Order in linked list takes O(1)
而家諗緊有冇可能可以用array而唔洗用linked list
