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Master system design problems that test data structure knowledge: LRU/LFU caches, Twitter feed, file systems, and 15+ design problems with full implementations.
Implement an LRU (Least Recently Used) cache with O(1) get and put using a doubly linked list and hashmap. Full 5-language solutions.
Implement an LFU cache with O(1) get and put. Uses two hashmaps and per-frequency doubly linked lists to track access frequency and recency simultaneously.
Design a simplified Twitter where users post tweets and follow each other, with getNewsFeed returning the 10 most recent tweets from followed users using a min-heap merge.
Implement a file system that creates paths and associates values with them. Uses a Trie or HashMap to map full paths to values with O(L) operations.