dsa2 min read
Minimum Number of Refueling Stops [Hard] — Greedy Max-Heap
Find the minimum refueling stops to reach target by greedily picking the largest fuel station passed so far whenever we run out.
Read →
Heap
21 articles
dsa1 min read
Minimum Cost to Connect Sticks — Min-Heap Greedy
Minimum cost to connect all sticks. Always connect the two cheapest (Huffman coding variant). Min-heap greedy: cost = sum of merged sticks each round.
Read →
dsa2 min read
Design Twitter — HashMap + Heap for News Feed
Design Twitter with follow/unfollow and getNewsFeed using per-user tweet lists and a min-heap merge.
Read →
dsa3 min read
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.
Read →
dsa2 min read
Meeting Rooms II
Find minimum conference rooms required by tracking room end times with a min-heap of ongoing meeting endings.
Read →
dsa3 min read
Design Twitter
Design a simplified Twitter with follow/unfollow and a news feed showing the 10 most recent tweets from followed users.
Read →
dsa2 min read
Merge K Sorted Lists — Min-Heap or Divide and Conquer
Merge k sorted linked lists into one sorted list using a min-heap for O(n log k) time complexity.
Read →
dsa2 min read
Find Median from Data Stream — Two Heaps
Find the median dynamically as numbers are added using two heaps: a max-heap for the lower half and min-heap for upper.
Read →
dsa2 min read
Sliding Window Median [Hard] — Two Heaps with Lazy Deletion
Find the median of each sliding window of size k using two heaps (max-heap for lower half, min-heap for upper half) with lazy deletion.
Read →
dsa2 min read
Prim's Algorithm — MST via Min-Heap Greedy Expansion
Build the minimum spanning tree using Prim's algorithm: start from any vertex, greedily expand by always adding the minimum-weight crossing edge using a min-heap.
Read →
dsa2 min read
Dijkstra Advanced — Modified Dijkstra Patterns (K Stops, States, Constraints)
Advanced Dijkstra patterns: state-space Dijkstra for problems with extra constraints like K stops, fuel limit, or restricted nodes. The key is augmenting the state beyond just the node.
Read →
dsa2 min read
Swim in Rising Water — Dijkstra / Binary Search + BFS
Find the minimum time to swim from (0,0) to (n-1,n-1) where you can only move to a cell when time >= cell value. Two approaches: Dijkstra O(n² log n) and binary search + BFS O(n² log n).
Read →
dsa2 min read
Amazon — Kth Largest Element in a Stream (Min-Heap)
Design a stream that always returns the kth largest element after each add. Uses a min-heap of size k — the top is always the kth largest.
Read →
dsa2 min read
Amazon — Task Scheduler (Greedy + Frequency Heap)
Find the minimum intervals to complete all tasks given cooldown n. Greedy with max-heap: always execute most frequent available task, idle only when forced.
Read →
dsa2 min read
Amazon — Merge K Sorted Lists (Min-Heap)
Merge k sorted linked lists into one sorted list using a min-heap. O(n log k) time by always extracting the current minimum across all list heads.
Read →
dsa2 min read
Amazon — Top K Frequent Elements (Bucket Sort / Heap)
Find the k most frequent elements using a max-heap or bucket sort. O(n) bucket sort approach using frequency as bucket index — faster than O(n log n) heap.
Read →
dsa2 min read
Google — Trapping Rain Water II (3D BFS + Min-Heap)
Calculate how much water can be trapped in a 3D height matrix. Uses a min-heap BFS starting from the boundary, always processing the lowest border cell to determine water trapped inside.
Read →
dsa4 min read
Design Twitter — Social Media Feed with Top K Posts
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.
Read →
dsa3 min read
Design Search Autocomplete System — Trie with Frequency Ranking
Design a search autocomplete system that returns top 3 historical queries matching the current prefix, sorted by frequency then lexicographically. Uses a Trie with per-node frequency maps.
Read →
dsa2 min read
Design a Leaderboard — Score Tracking with Top K
Design a leaderboard that tracks player scores, supports score additions, top K sum queries, and player resets using a hashmap with sorted aggregation.
Read →
dsa2 min read
Find Median from Data Stream — Two-Heap Approach
Maintain a running median from a data stream using two heaps: a max-heap for the lower half and a min-heap for the upper half, rebalancing after each insertion.
Read →