Implemented LRU Cache using unordered_map + list

[sachiny123]

This PR implements the Least Recently Used (LRU) Cache data structure in C++ using
unordered_map and list for efficient O(1) access and updates.

Features:

• get(key): Returns the value of the key if it exists, else returns -1.
• put(key, value): Inserts a new key-value pair or updates an existing key.
  If the cache exceeds its capacity, the least recently used item is removed.
• Efficient O(1) time complexity for both get and put operations.
• Includes example usage in main() demonstrating functionality.

This implementation is practical for interview preparation and real-world applications
where caching with eviction policy is required.

[vil02]

No code => no bugs 😄

[mindaugl]

It is truly a perfect implementation.