Bit rank, depot paths, and sorted-run contracts

Use the exact range and freshness contract in each question. One off-by-one bit, repeated ancestor, or ignored tombstone changes the answer.

Lessons

• Bitvector rank and select: count and locate set bits
• Heavy-light decomposition: sum weights along a tree path
• Sorted runs and tombstones: model an LSM read path

Design exercise

• Project: audit incident flags, depot paths, and sorted runs
• Data Structures