Update problem 61 & 87

[joyemang33]

Summary

Before this PR, problems 61 and 87 could be reward hack due to some evaluator bugs. This PR fixed them and add more public tests for problem 61.

Type of Change

• New research problem
• New algorithmic problem
• Bug fix
• Documentation update
• Other:

Testing

Checklist

• Code follows the project structure and conventions
• Self-review completed
• Documentation updated (if applicable)

CI Validation (for new problems)

When adding new problems, CI will automatically validate that your reference solution achieves score > 0.

• Algorithmic problems: Include reference.cpp in your problem directory
• Research problems: Include reference.py (or reference.cpp if language: cpp in config.yaml)