Return the length of the longest contiguous substring forming valid parentheses.
A good solution should
- State the invariant or decision rule that makes the approach correct.
- Explain time and space costs in terms of input size and required output.
- Handle empty, minimal, duplicate, and boundary-shaped inputs.
Reasoning prompt
Compare index-stack sentinels with two directional counter scans. Unmatched closing and opening brackets reset different boundaries.
Use the linked judge for the canonical challenge. The examples above are compact TRT checks; create additional tests around the boundary most likely to break your invariant.