Arrays, Hashing, and Design

Nothing in this section is a Pinterest invention. Two Sum, Group Anagrams, a Premium question, Product of Array Except Self, and a short shelf of design classics, LRU cache, insert-delete-getRandom in O(1), a min stack, a hit counter. Standard FAANG fare, medium-centered, the same bank you would drill for any big-tech loop. What Pinterest adds is product framing: the integers become Pins, the generic id router becomes an experiment-bucket allocator. The label changes while the pattern holds. Treat it that way.

So the honest target here is fluency on familiar patterns. One idea carries most of the array-and-hashing problems: a hash map that turns "have I seen something that completes this?" from an inner loop into a constant-time lookup. Get that reflex solid and Two Sum, the anagram buckets, and the subarray-count problems all collapse into a single pass. The design problems run on a sister idea. Pick the data structure that makes the one expensive operation cheap. Both halves are below.

The Pinterest angle

The interviewer is going to push you off the brute force, so arrive already past it. Reports out of Pinterest's CoderPad rounds are consistent: a first answer is fine, then "can we do better on time?" lands within a minute. For the array problems that usually means a hash map standing in for a nested loop. O(n) instead of O(n squared). Say the complexity of your first idea before you write it, name the structure that beats it, then write the better one. Defending a quadratic scan is the avoidable way these go sideways.