dsa2 min read
Sort Colors [Medium] — Dutch National Flag Algorithm
Sort an array of 0s, 1s, and 2s in-place in one pass using Dijkstra's Dutch National Flag algorithm.
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27 articles
dsa2 min read
Subsets [Medium] — Backtracking & Bit Manipulation
Generate the power set of distinct integers using backtracking or bitmask enumeration.
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Increasing Triplet Subsequence [Medium] — Greedy Two Trackers
Determine if there exists an increasing subsequence of length 3 using two greedy variables in O(n) time O(1) space.
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dsa2 min read
Longest Consecutive Sequence [Medium] — HashSet O(n)
Find the longest consecutive integer sequence in O(n) using a HashSet and only extending sequences from their start.
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Task Scheduler [Medium] — Greedy Frequency Buckets
Calculate minimum CPU intervals for tasks with cooldown using a greedy formula based on the most frequent task count.
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dsa1 min read
Maximum Swap [Medium] — Rightmost Digit Tracking
Maximize a number by making at most one swap, using a last-occurrence map to find the optimal swap greedily.
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Find Peak Element [Medium] — Binary Search on Slope
Find a peak element in O(log n) by binary searching on the slope direction — always move toward the higher neighbor.
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dsa2 min read
Trapping Rain Water [Hard] — Two Pointers O(n)
Calculate trapped rain water using inward two pointers that track left-max and right-max, replacing the classic O(n) space approach.
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Minimum Window Substring [Hard] — Sliding Window with Frequency Map
Find the minimum window in s that contains all characters of t using a shrinkable sliding window with a character frequency counter.
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Median of Two Sorted Arrays [Hard] — Binary Search on Partition
Find the median of two sorted arrays in O(log(min(m,n))) by binary searching for the correct partition point.
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Two Sum [Easy] — HashMap Complement Lookup
Find two indices that add to target in O(n) by storing each number in a HashMap and looking up the complement.
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dsa1 min read
Group Anagrams [Medium] — Sorted Key HashMap
Group strings that are anagrams together by using their sorted form as a HashMap key.
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Top K Frequent Elements [Medium] — Bucket Sort O(n)
Find the k most frequent elements in O(n) using bucket sort by frequency instead of a heap.
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LRU Cache [Medium] — Doubly Linked List + HashMap
Design a Least Recently Used cache with O(1) get and put operations using a HashMap combined with a doubly linked list.
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dsa1 min read
Subarray Sum Equals K [Medium] — Prefix Sum HashMap
Count subarrays with sum exactly k using prefix sums stored in a HashMap to find valid starting positions in O(n).
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Longest Consecutive Sequence [Medium] — HashSet O(n)
Find the longest consecutive integer sequence in O(n) by using a HashSet and only starting sequences from their minimum element.
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dsa2 min read
Insert Delete GetRandom O(1) [Medium] — Array + HashMap Design
Design a set with O(1) insert, delete, and getRandom using a dynamic array combined with a HashMap for index tracking.
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Remove Duplicates from Sorted Array [Easy] — Write Pointer
Remove duplicates in-place from a sorted array using a write pointer that only advances on unique elements.
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Longest Substring Without Repeating Characters [Medium] — HashSet Window
Find the longest substring without duplicate characters using a HashSet-backed shrinkable sliding window.
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dsa1 min read
Container with Most Water [Medium] — Greedy Two Pointers
Maximize water trapped between two vertical lines using inward two pointers that always move the shorter line inward.
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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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Max Consecutive Ones [Medium] — K-Flip Sliding Window
Find the maximum consecutive ones in a binary array when you can flip at most k zeros, identical to the Max Consecutive Ones III pattern.
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dsa2 min read
Minimum Window Substring [Hard] — have/need Frequency Counter
Find the smallest window in s containing all characters of t using a have/need counter to track when the window is valid.
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Longest Substring with At Most Two Distinct Characters [Hard]
Find the longest substring containing at most 2 distinct characters using a variable sliding window with a character count map.
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Sort Colors / Dutch National Flag [Medium] — 3-Pointer Review
Sort an array of 0s, 1s, and 2s in one pass using the Dutch National Flag 3-pointer algorithm.
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Valid Palindrome II [Medium] — Delete At Most One Character
Check if a string can become a palindrome by deleting at most one character using recursive two-pointer palindrome checking.
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Trapping Rain Water [Hard] — Two Pointer Classic
Calculate trapped rainwater using optimal two-pointer approach that tracks left and right maximums without extra space.
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