Arrays, Strings and Hashing

Arrays, strings, and hash maps are the three data structures you will touch in almost every coding round, and a hash map is the single most useful tool you can bring to one. Most “make this faster” follow-ups in a quant interview are really “replace this nested loop with a dictionary lookup.” This lesson covers how Python lists actually store data, why strings being immutable changes how you build them, and why a dict or set turns an $O(n)$ scan into an $O(1)$ lookup. Then it works four of our interview questions end to end: grouping anagrams, checking palindromes, counting zero-sum subarrays with prefix sums, and counting word frequencies.