Arrays, Hashing & Strings

Most Amazon loops open here. Medium problems, no Run button, and an interviewer grading two things at once: did you land on the right data structure, and can you explain why it works. A hash map or a sliding window collapses an O(n²) brute force into one clean sweep. Show that collapse and you've banked most of the points on offer.

Two ideas cover almost all of it. A hash map answers "have I seen this key, and where" in constant time, which kills nested-loop scans. A sliding window keeps a stretch of the array under a rule by moving two pointers, so you re-read each element once instead of re-scanning every subarray. Learn those two. The rest is bookkeeping.