Check ordered ranks, grid corrections, and majority windows

Choose the operation contract or failure response implied by each structure. Check each choice against the runnable trace.

Lessons

• Ordered treaps: split, join, and select depot keys
• Span skip lists: rank and select without a full scan
• Two-dimensional segment trees: correct sensors and sum rectangles
• Range-majority indexes: verify a candidate before returning it

Design exercise

• Project: audit ordered ranks, grid corrections, and majority windows
• Data Structures