What is the hash map pattern and when should you use it in coding interviews?

The hash map pattern uses a dictionary or hash table to store values for constant-time lookups, turning brute-force quadratic solutions into linear-time solutions. Use it whenever you need to check if an element exists, count frequencies, or find pairs that satisfy a condition. The classic example is the two-sum problem: instead of checking every pair, store each number in a hash map and check if the complement exists.