Design Skiplist

Implement a skiplist without using any built-in ordered containers or libraries. A skiplist stores sorted values in several layers of linked lists, where each higher layer skips over more nodes, so that searching, inserting, and deleting take O(log n) time on average.

Implement the Skiplist class:

• Skiplist() creates an empty skiplist.
• bool search(int target) returns true if target is present at least once, otherwise false.
• void add(int num) inserts num. Duplicate values are allowed and each copy is stored separately.
• bool erase(int num) removes one copy of num and returns true; if num is not present, it does nothing and returns false.

The input lists the method names and their arguments; the output lists the return value of each call, with null for the constructor and for add.