• Write an iterator class encapsulating in-order traversal of a binary tree. What is its time complexity? What is the worst case?

• You have a collection of HTML documents grouped by a source website. You need to count frequencies of bigrams across all documents and present them in sorted order (perhaps by returning the top N frequent bigrams). How would you approach this? How would you filter out non-significant bigrams? How would you distribute and merge the results? What data structures would you use to keep counters?

• OOP: Write classes representing an expression tree for a simple calculator. You care only about constants and basic binary operators. Write a function for evaluating expressions.

• N clients and M servers. Each server can handle a limited number of queries per second. How would you improve availability and performance?

• Write code to shift a string with rotation. Example: [a, b, c << 2 = c, a, b].

• How to count character frequencies in a string? What if the string is too huge? When is it reasonable to distribute this task across machines? How to find the most frequent character in a distributed scenario with minimum data exchange between machines?

• You are given a huge stream of unordered geo-coordinates. Return a list of objects within a specified radius from some point X.

Count bi-grams in a huge collection of documents, grouped by source web site.