Dinosaur Speed (Meta PE classic)

Meta Production Engineer classic. Given timestamps when dinosaurs cross sensor lines, pair them up so each pair represents the same dinosaur, and compute speeds. Time-window pairing + binary search. Requirements Two sensor lines a fixed distance apart; each line records timestamps when something crosses it. A "dinosaur" pair = a crossing at line 1 followed by a crossing at line 2 within a reasonable window. Pair them greedily by minimum time delta, compute speed for each pair. Follow-ups: filter on a speed threshold, handle multiple dinosaurs in flight, return only pairs that exceed N mph. Examples Line A: [1, 5, 10]; Line B: [2, 6, 12]; distance = 100. Pairs: (1,2), (5,6), (10,12); speeds 100/1, 100/1, 100/2. Notes PE-track-specific — this prompt is rarely seen outside Production Engineer loops. Time-window pairing is the standard pattern; binary-search the matching candidate within [t + min_dt, t + max_dt]. Common bug: O(n²) brute-force pairing when O(n log n) two-pointer / binary-search is expected. Preparation Write the binary-search version from memory in under 12 min. Practice the speed-threshold follow-up (filter + return list). Pair with the standard PE phone-screen partner problem: top-N frequent words from a file.