Chandan

Implements the Dutch National Flag algorithm to sort an array of 0s, 1s, and 2s in-place. Includes error handling for invalid values and provides examples in the docstring. ### Describe...

## Summary of your changes ### Description Adds USACO.guide to the "Competitive programming" section. This is a structured, level-based training platform for competitive programming and Olympiads. This pull request follows...