Easy
First Unique Character:Count Once, Then Walk for the First One
Count frequencies, then return the first index whose count is 1. Nested uniqueness scans are honest and quadratic.
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9 questions
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- Easy
Happy Number:A Set Stops the Cycle the Unbounded Loop Misses
A HashSet of seen sums proves you are looping, not still searching for 1. An unbounded digit-square walk hangs on the unhappy cycle; Floyd is the space follow-up.
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Ransom Note:Spend the Magazine Counts, Do Not Scratch Each Letter
Magazine letter counts must cover the note. Nested scratch-and-scan is honest and quadratic; anagram equality is the wrong test.
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Word Pattern:Pair Letters to Tokens Both Ways, Not One Map
A bijection between pattern letters and words. Nested consistency scans are honest and quadratic; a one-way map lets two letters share a token.
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- Medium
LRU Cache:Hash the Node, Then Promote It
O(1) get and put need a HashMap to the node plus a doubly linked recency list. Scanning pairs to move-to-end is correct and linear.
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Randomized Set:Hash the Slot, Swap-Last on Delete
A HashSet cannot pick a uniform member in O(1). Pair an ArrayList with a value-to-index map so delete can swap-with-last and getRandom is an index.
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Subarray Sum Equals K:Hash the Prefix Complement, Not the Nested Window
Count windows that sum to k by hashing how often each prefix appeared. Nested i..j is honest and quadratic; a sliding window fails the moment a refund is legal.
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Time-Based Store:Hash the Key, Probe As-Of
Timestamps arrive in order per key, so set is an append and get is a rightmost stamp still legal at t. Scanning that history is honest and too slow.
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Top K Frequent:Count Once, Then Evict the Quietest
HashMap counts plus a size-k min-heap beat nested next-hottest scans. Bucket sort is the O(n) follow-up, not the usual board default.
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