Add popular LeetCode problems with solutions

Implement solutions to popular LeetCode problems across various categories:

## Easy Problems (5-10)
- Two Sum
- Valid Parentheses
- Merge Two Sorted Lists
- Maximum Subarray
- Climbing Stairs

## Medium Problems (5-10)
- Add Two Numbers
- Longest Substring Without Repeating Characters
- 3Sum
- Letter Combinations of a Phone Number
- Generate Parentheses

## Hard Problems (3-5)
- Median of Two Sorted Arrays
- Regular Expression Matching
- Merge k Sorted Lists
- Trapping Rain Water
- LRU Cache Implementation

Each solution should:
- Include problem statement and constraints
- Provide a well-commented implementation in Hy
- Include complexity analysis
- Have comprehensive test cases
- Compare different solution approaches where applicable

This collection will demonstrate Hy's capabilities for solving common algorithmic problems and provide reference implementations.