Arrays, Strings & Hashing

This is the warm-up band, and at DoorDash it owns the phone screen. Arrays, strings, hashing, and binary search are where a screen starts and where the new-grad HackerRank assessment lives. The problems read as Medium, sometimes Easy, and the bar is whether your code runs against the test cases on the first or second try while the interviewer watches.

Three problems show up again and again in DoorDash phone-screen reports, and they map cleanly onto three skeletons. A Premium question, an expression you evaluate with a stack. Koko Eating Bananas, which DoorDash reframes as a delivery-capacity question and you solve by binary-searching the answer. A Premium question, a textbook sliding window. Learn those three shapes cold and most of this band falls out.