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4Sum II — Two-Map Split and Match
Count 4-tuples summing to zero by splitting into two pairs and using a frequency map of pair sums.
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LFU Cache — HashMap + Frequency Buckets + DLL
Implement an LFU cache with O(1) get/put using three hash maps: key→val, key→freq, freq→OrderedDict.
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All O'one Data Structure — DLL of Buckets + HashMap
Design a data structure supporting inc/dec and getMaxKey/getMinKey in O(1) using a doubly linked list of frequency buckets.
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Design Twitter — HashMap + Heap for News Feed
Design Twitter with follow/unfollow and getNewsFeed using per-user tweet lists and a min-heap merge.
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Palindrome Pairs — HashMap + Palindrome Check
Find all index pairs forming palindromes by checking prefix/suffix splits against a reverse-word map.
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