Add Java implementation for LFU (Least Frequently Used) Cache

[priyashuu]

This PR adds a Java implementation of an LFU Cache that supports both get() and put() operations in O(1) average time complexity.
The implementation efficiently handles eviction of the least frequently used elements, breaking ties using Least Recently Used (LRU) order.

Key Features:

• Efficient O(1) get() and put() operations.
• Automatically evicts the least frequently used key on capacity overflow.
• Maintains LRU order among nodes with the same frequency.
• Uses HashMap + LinkedHashSet for quick lookup and ordering.

Example:

Input:

Output:

Complexity Analysis:

Time Complexity: O(1) average for get() and put()

Space Complexity: O(capacity)

Files Added:

LFUCache.javaa complete implementation with comments, test example, and explanation.

[github-actions]

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[github-actions]

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✅ Passed - All code compiles successfully!

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