When should you use a hash map?

Here's the signal: you catch yourself searching for something you've already seen. "Where did this number appear before?", "how many times have I met this letter?", "is its partner already here?" — each of those is a nested loop waiting to become a single map lookup. Almost every hash map question in interviews uses one of four patterns.

1. Counting

Whenever a question asks how many times, how many different, or which is most common, keep a map from each item to its count. One pass builds it, and every later question is a lookup. You'll use this in *Count Distinct Elements in Every Window* and *Check If Array Pairs Are Divisible by K*.