Array & String Foundations

• Arrays are contiguous: optimize by indexing and in-place updates.
• Invariants drive correctness: define what stays true before coding.

Core patterns

• Read/Write compaction (remove, dedupe)
• Two pointers from ends (merge, swap, symmetry)
• Rotation via reverse (3-step reverse trick)
• Single-pass greedy scans (min so far / farthest reach)
• Counting and selection (Boyer-Moore, H-index buckets)
• Prefix/Suffix products (exclude self)

Common pitfalls

• Off-by-one on empty or single-element arrays
• Overwrite bugs when merging from the front
• Forgetting k %= n for rotation
• Returning a new slice instead of new length