Counter technique is simply using same old dict object in Python to solve the problems. There is also specific
subclass of dict for this matter as collections.Counter. The key point of this technique is not how to use
counters but rather how to develop an algorithm which uses counters to solve the problem efficiently. Therefore
related problems are going to be much more explanatory for this topic.
HackerRank - Count Triplets
HackerRank - Frequency Queries
HackerRank - Largest Rectangle
HackerRank - Sherlock and the Valid String
HackerRank - Special String Again
https://docs.python.org/3/library/collections.html#collections.Counter