Design a class to implement a stack using only a single queue. Your class, QueueStack, should support the following stack methods: push() (adding an item), pop() (removing an item), peek() (returning the top value without removing it), and empty() (whether or not the stack is empty).
This question is asked by Microsoft. Implement a trie class that supports insertion and search functionalities.
This question is asked by Microsoft. Design a class, MovingAverage, which contains a method, next that is responsible for returning the moving average from a stream of integers.
This question is asked by Google. Create a class CallCounter that tracks the number of calls a client has made within the last 3 seconds. Your class should contain one method, ping(int t) that receives the current timestamp (in milliseconds) of a new call being made and returns the number of calls made within the last 3 seconds.