Easy
Kth Largest Stream:Keep k Winners, Peek the Floor
A size-k min-heap peeks the kth largest after each add. Sorting the whole stream on every arrival is honest; a max-heap of every number ranks losers you will never return.
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11 questions
- Medium
Design Twitter:Heap of Timeline Tails, Not a Graph Walk
Each user’s tweets are an append-only list; a max-heap of those tails is merge-k. Collect-and-sort is honest; BFS on the follow graph is the wrong shape.
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K Closest Points:Keep k Winners, Evict the Farthest
A size-k max-heap on squared distance polls the farthest of the current winners. Sorting every pin ranks depots you will never dispatch.
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K Smallest Pairs:Seed One Axis, Expand the Other
Each nums1 index against nums2 is a sorted list of pair sums. A min-heap of those heads is merge-k; expanding both neighbors from (0, 0) needs a seen-set that one-axis seed avoids.
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Kth Largest Array:Keep k Winners Once, Peek the Floor
A size-k min-heap peeks the kth largest of a frozen array. Sorting descending is honest; ranking every loser you throw away is the extra bill.
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Merge K Sorted Lists:Heap of Heads, Splice onto a Dummy Tail
A min-heap of current heads plus dummy-tail splice rewires k sorted lists in place. Dumping values and sorting rebuilds a new chain; pairwise two-list merge is the heap-free alternative.
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Reorganize String:Poll the Loudest, Hold the Previous
A max-heap of remaining counts plus a one-slot hold beats trying rearrangements. Idle is failure here — if the only leftover is the letter you just placed, return empty.
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Task Scheduler:Poll the Loudest, Park It Until Ready
A max-heap of remaining counts plus a cooldown queue beats rescanning leftovers each slot. The idle-gap formula is the follow-up, not the board default when they said heap.
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Top K Frequent Words:Count Once, Rank Ties, Drain Then Reverse
Unlike numeric Top K, the k words must come back frequency-desc then lex-asc. A size-k min-heap evicts the quietest, or the later dictionary word on a tie; drain then reverse.
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Representation and operations — not the problem set.