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Kth Largest Element in a Stream — Min-Heap Design [Amazon Easy]
Design a class to find the kth largest element in a data stream. Min-heap of size k: the top is always the answer. Full C++, Java, JavaScript and Python.
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Heaps
47 articles
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Heaps & Priority Queues — Complete Interview Guide
Master heaps and priority queues: 7 core patterns including Top K, Two Heaps, Merge K, and scheduling problems.
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Kth Largest Element in a Stream
Maintain a min-heap of size K to track the Kth largest element as numbers are added to a stream.
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Last Stone Weight
Simulate stone smashing by repeatedly extracting the two heaviest stones using a max-heap.
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Relative Ranks
Assign gold/silver/bronze or rank numbers to athletes based on their scores using sorted ordering.
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Find K Closest Elements
Find K closest elements to x in a sorted array using binary search to find the optimal left boundary.
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Kth Largest Element in an Array
Find the Kth largest element using quickselect O(n) average or a min-heap of size K in O(n log k).
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Top K Frequent Elements
Return the K most frequent elements using a frequency map plus min-heap, or bucket sort for O(n) solution.
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Sort Characters By Frequency
Sort characters in a string by decreasing frequency using a frequency counter and max-heap or bucket sort.
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K Closest Points to Origin
Find K points closest to the origin using a max-heap of size K or quickselect for O(n) average.
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Task Scheduler
Find minimum CPU intervals to schedule tasks with cooldown using a greedy max-heap simulation.
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Reorganize String
Rearrange a string so no two adjacent characters are the same using a greedy max-heap approach.
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Find Median from Data Stream
Maintain a dynamic median using two heaps: a max-heap for the lower half and a min-heap for the upper half.
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Sliding Window Median
Compute medians of all sliding windows using two heaps with lazy deletion for element removal.
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Merge K Sorted Lists
Merge K sorted linked lists into one sorted list using a min-heap to always extract the globally smallest node.
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Kth Smallest Element in a Sorted Matrix
Find the Kth smallest element in an n×n row-and-column sorted matrix using binary search on value range or heap.
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Find K Pairs with Smallest Sums
Find K pairs (u, v) with smallest sums from two sorted arrays using a min-heap initialized with first row candidates.
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Ugly Number II
Find the nth ugly number (only prime factors 2, 3, 5) using three pointers for the 2x, 3x, 5x merge streams.
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Course Schedule III
Maximize courses taken within deadlines by greedily scheduling and swapping out the longest past course.
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Meeting Rooms II
Find minimum conference rooms required by tracking room end times with a min-heap of ongoing meeting endings.
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Minimum Cost to Connect Sticks
Find minimum cost to connect all sticks by greedily always merging the two shortest sticks first.
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IPO
Maximize capital after k IPO investments by greedily picking the highest profit project among affordable ones.
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Car Pooling
Check if a car can pick up all passengers by processing start/end events with a sorted timeline or heap.
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Furthest Building You Can Reach
Greedily use ladders for the largest climbs and bricks for smaller gaps, managed with a min-heap of ladder sizes.
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Process Tasks Using Servers
Assign tasks to available servers using two heaps: one for free servers and one for busy servers sorted by free time.
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Trapping Rain Water II
Calculate trapped water in a 3D height map using BFS with a min-heap to process cells from shortest boundary inward.
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Swim in Rising Water
Find minimum time to swim from top-left to bottom-right in a grid where time = max elevation on the path.
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Smallest Range Covering Elements from K Lists
Find the smallest range that contains at least one element from each of K sorted lists using a sliding K-way merge.
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Super Ugly Number
Find the nth super ugly number with prime factors only from a given list using K-pointer dynamic programming.
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Minimum Number of Refueling Stops
Find minimum fuel stops to reach the target by greedily selecting the largest available fuel stations when running low.
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Design Twitter
Design a simplified Twitter with follow/unfollow and a news feed showing the 10 most recent tweets from followed users.
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Ugly Number III
Find the nth number divisible by a, b, or c using binary search with inclusion-exclusion counting formula.
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Maximum Performance of a Team
Maximize team performance (sum of speeds * min efficiency) by sorting by efficiency and using a min-heap on speeds.
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Longest Happy String
Build the longest string with at most 2 consecutive identical characters by always using the most frequent available character.
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Maximum Frequency Stack
Design a stack that pops the most frequent element, breaking ties by most recently pushed, using frequency-bucket stacks.
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Minimize Deviation in Array
Minimize max-min of an array by doubling odd numbers and repeatedly halving the max using a max-heap.
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Median of K Sliding Windows (Extended Practice)
Extended practice: compute median for each window in a stream using two-heap with lazy deletion pattern.
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Minimum Interval to Include Each Query
For each query point, find the smallest interval containing it by sorting both intervals and queries, using a min-heap.
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Find Right Interval
For each interval find the interval with the smallest start point >= the end point using sorted starts and binary search.
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Sort Array by Increasing Frequency
Sort array elements by frequency ascending, breaking ties by value descending.
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Kth Largest Sum in a Binary Tree
Find the kth largest level sum in a binary tree using BFS to compute level sums then sorting or using a heap.
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Total Cost to Hire K Workers
Minimize total hiring cost by always choosing the cheapest candidate from the first or last p candidates using two min-heaps.
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Seat Reservation Manager
Design a seat manager that reserves the lowest numbered available seat and unreserves seats using a min-heap.
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Stock Price Fluctuation
Track stock prices with corrections by maintaining a sorted multiset and a timestamp-to-price map.
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Reduce Array Size to At Least Half
Find the minimum number of elements to remove to reduce array size by half by greedily removing most frequent elements.
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Find Subsequence of Length K With the Largest Sum
Find the subsequence of length k with the largest sum by selecting top k values while preserving original order.
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Heaps & Priority Queues — Master Recap & Cheatsheet
Complete Heaps section recap with 7 patterns, complexity guide, and full problem index for interview prep.
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