You are given a complete binary tree. A complete binary tree is one where every level, except possibly the last, is completely filled, and all nodes are as far left as possible. Your task is to design a data structure that supports inserting new nodes while maintaining the completeness of the binary tree. Implement the CBTInserter class that supports two operations: inserting a new node and returning the root of the tree.
Design a time-based key-value data structure that stores multiple values for the same key at different timestamps and supports retrieving the value at a specific timestamp.
Design a CombinationIterator class that generates all combinations of a specified length from a sorted string of distinct lowercase English letters, and allows iterating through them in lexicographical order.
A social media company wants to analyze tweet activity by counting the number of tweets in given time periods, broken down by minute, hour, or day frequency. Design an API to record tweets and calculate the number of tweets within specific time intervals.