dsa2 min read
Third Maximum Number — OrderedSet O(n) [Microsoft Easy]
Return the third distinct maximum or the max if fewer than 3 distinct values exist. Three-variable tracking O(n) O(1) solution.
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Sorting
20 articles
dsa14 min read
3Sum — The Deduplication Problem Every FAANG Interviewer Loves [LC 15]
Learn why 3Sum is a FAANG interview staple — how sorting enables two pointers, why deduplication trips up even strong candidates, a full visual dry run, and every common bug explained with Python and JavaScript solutions.
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dsa12 min read
Merge Intervals — The Definitive Guide (LeetCode 56) [Google, Meta, Amazon, Microsoft]
Master the classic Merge Intervals problem (LeetCode 56) asked at Google, Meta, Amazon, and Microsoft. Learn why sorting by start time is the key insight, the exact overlap condition (c ≤ b), a step-by-step visual dry run, 4 common mistakes, Python and JavaScript solutions, and follow-up problems including Insert Interval, Non-overlapping Intervals, and Meeting Rooms II.
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dsa23 min read
Top K Frequent Elements — Three Approaches from O(n log n) to O(n) [Google, Amazon, LinkedIn]
LeetCode 347 asks for k most frequent elements with a constraint: beat O(n log n). Learn why naive sort fails, how a min-heap of size k achieves O(n log k), and the elegant bucket sort insight that delivers true O(n) — with full visual dry run, common mistakes, and Python/JavaScript solutions for all three approaches.
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dsa1 min read
Wiggle Sort II [Medium] — Virtual Index Mapping
Rearrange an array so nums[0] `<` nums[1] > nums[2] `<` nums[3]... using virtual index mapping and nth_element for O(n) time.
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dsa2 min read
Advantage Shuffle [Medium] — Greedy Strongest Beat Weakest
Rearrange nums to maximize advantage over B using a greedy strategy: assign the smallest winning card, else discard the smallest.
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dsa1 min read
Shortest Unsorted Continuous Subarray [Medium] — Linear Scan
Find the shortest subarray that when sorted makes the whole array sorted, using a single linear scan tracking violated boundaries.
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dsa1 min read
Successful Pairs of Spells and Potions — Sort + Binary Search
Count spell-potion pairs with product >= success by sorting potions and binary searching for each spell's threshold.
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dsa1 min read
Russian Doll Envelopes — Sort + LIS Binary Search
Find the maximum number of nested envelopes by sorting by width ascending then height descending, then applying LIS.
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dsa2 min read
Relative Ranks
Assign gold/silver/bronze or rank numbers to athletes based on their scores using sorted ordering.
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dsa2 min read
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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dsa1 min read
Sort Array by Increasing Frequency
Sort array elements by frequency ascending, breaking ties by value descending.
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dsa1 min read
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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dsa2 min read
Vertical Order Traversal of a Binary Tree
Group tree nodes by vertical column then row, sorting by value within same position using BFS with coordinates.
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dsa3 min read
Minimum Number of Operations to Sort Binary Tree by Level
Find minimum swaps to sort each level of a binary tree using BFS and cycle-detection in permutation sorting.
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dsa2 min read
3Sum [Medium] — Sort + Two Pointer Scan
Find all unique triplets summing to zero by sorting and using two pointers for each fixed element with careful duplicate skipping.
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dsa1 min read
Frequency of the Most Frequent Element [Medium] — Sorted Window
Find the maximum frequency of any element after at most k increments by sorting and using a sliding window with a running sum.
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dsa1 min read
Minimum Difference Between Highest and Lowest of K Scores [Easy] — Sort + Fixed Window
Find the minimum difference between the max and min of any k scores by sorting and using a fixed window of size k.
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dsa2 min read
Suffix Array — O(n log n) Construction and Applications
Build a suffix array in O(n log²n) using doubling sort. Enables O(m log n) substring search, longest common substring, and number of distinct substrings.
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dsa3 min read
Design In-Memory Database — Multi-Field Filtering
Design an in-memory key-value database that supports set, get, delete, and rank operations. Demonstrates combining hashmaps with sorted structures for efficient multi-key queries.
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