dsa2 min read
Max Consecutive Ones III — Sliding Window with K Flips [Google, Meta Medium]
Find maximum consecutive ones if you can flip at most k zeros to one. Variable sliding window O(n) solution.
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Sliding-window
75 articles
dsa2 min read
Longest Substring Without Repeating Characters — Sliding Window [Google, Amazon, Meta]
Find length of longest substring without repeating characters. Optimal O(n) sliding window with HashMap tracking last seen positions.
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dsa2 min read
Minimum Size Subarray Sum [Medium] — Sliding Window
Find the smallest contiguous subarray with sum >= target using the shrinkable sliding window technique.
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dsa2 min read
Sliding Window Maximum [Hard] — Monotonic Deque
Find the maximum in every sliding window of size k in O(n) using a monotonic decreasing deque.
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dsa2 min read
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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dsa2 min read
Maximum Sum of 3 Non-Overlapping Subarrays [Hard] — Sliding Window + DP
Find three non-overlapping subarrays of length k with maximum sum using sliding window sums and left/right best index arrays.
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dsa1 min read
Longest Subarray of 1s After Deleting One Element [Medium] — Sliding Window
Find the longest subarray of 1s after deleting exactly one element using a sliding window that tracks the count of zeros.
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dsa1 min read
Subarrays with K Different Integers [Hard] — Sliding Window Subtract
Count subarrays with exactly K distinct integers using the formula: atMost(K) - atMost(K-1).
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dsa2 min read
Find All Anagrams in a String [Medium] — Fixed Window + Freq Match
Find all starting indices of anagram substrings by using a fixed window with character frequency comparison.
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dsa2 min read
Sliding Window Maximum — Monotonic Deque
Find the maximum in each sliding window of size k using a monotonic decreasing deque storing indices.
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dsa7 min read
Two Pointers & Sliding Window — Complete Guide (Problems 101–160)
Master Two Pointers and Sliding Window patterns with 60 problems, 5-language solutions, and MAANG company tags.
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dsa2 min read
Valid Palindrome [Easy] — Two Pointers Inward
Check if a string is a palindrome after removing non-alphanumeric characters using inward two pointers.
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dsa1 min read
Reverse String [Easy] — Two Pointers In-Place Swap
Reverse an array of characters in-place using the classic two-pointer swap technique.
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dsa2 min read
Squares of Sorted Array [Easy] — Two Pointers Fill from End
Return squares of a sorted array in sorted order using two pointers that compare absolute values from both ends.
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dsa1 min read
Remove Element [Easy] — Read/Write Two Pointers
Remove all occurrences of val in-place using a write pointer pattern, returning the new length.
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dsa2 min read
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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dsa2 min read
Merge Sorted Array [Easy] — Three Pointers From End
Merge two sorted arrays in-place from the end to avoid overwriting elements using three pointers.
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dsa2 min read
Is Subsequence [Easy] — Two Pointer Greedy Match
Check if string s is a subsequence of t using a greedy two-pointer scan that matches characters in order.
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dsa2 min read
Max Consecutive Ones III [Medium] — Sliding Window with K Zeros
Find the maximum number of consecutive 1s achievable by flipping at most k zeros, using a variable sliding window.
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dsa2 min read
Diet Plan Performance [Easy] — Fixed Sliding Window
Count days with fixed-size window sums above upper and below lower thresholds using a sliding fixed window.
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dsa1 min read
Running Sum of 1D Array [Easy] — Prefix Sum
Compute the running sum of a 1D array where each element is the sum of itself and all previous elements.
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dsa2 min read
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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dsa2 min read
Longest Repeating Character Replacement [Medium] — Max-Freq Window
Find the longest substring with at most k character replacements by tracking the maximum frequency char in the window.
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dsa2 min read
Permutation in String [Medium] — Fixed Window Character Count
Check if any permutation of s1 is a substring of s2 using a fixed-size sliding window with character frequency comparison.
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dsa2 min read
Fruit Into Baskets [Medium] — Longest Subarray with 2 Distinct
Find the maximum fruits you can collect starting from any tree with two baskets (at most 2 distinct fruit types) using a sliding window.
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dsa1 min read
Maximum Erasure Value [Medium] — Sliding Window with HashSet
Find the maximum sum of a subarray with all unique elements using a sliding window backed by a HashSet.
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dsa1 min read
Minimum Size Subarray Sum [Medium] — Shrinkable Sliding Window
Find the smallest subarray with sum >= target using a variable sliding window that shrinks from the left when the condition is met.
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dsa1 min read
4Sum II [Medium] — Split and Hash
Count tuples (i,j,k,l) where nums1[i]+nums2[j]+nums3[k]+nums4[l]=0 by hashing all pairs from first two arrays.
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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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dsa1 min read
Boats to Save People [Medium] — Greedy Two Pointers Sort
Find minimum boats to save everyone where each boat carries at most 2 people with total weight <= limit, using sort + greedy two pointers.
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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
Two Sum II — Input Array Is Sorted [Medium] — O(1) Space Two Ptr
Find two numbers summing to target in a 1-indexed sorted array using inward two pointers in O(n) time O(1) space.
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dsa1 min read
Subarray Product Less Than K [Medium] — Sliding Window
Count contiguous subarrays with product < k using a sliding window that divides from the left when product exceeds the limit.
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dsa1 min read
Maximum Points You Can Obtain from Cards [Medium] — Complement Window
Maximize card points taken from both ends of an array by finding the minimum-sum subarray of size n-k in the middle.
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dsa1 min read
Count Number of Nice Subarrays [Medium] — atMost(k)−atMost(k−1)
Count subarrays with exactly k odd numbers using the at-most(k) minus at-most(k-1) sliding window formula.
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dsa1 min read
Binary Subarrays With Sum [Medium] — Prefix Sum / atMost
Count binary subarrays with exactly the given sum using prefix sum with HashMap or the atMost(k)-atMost(k-1) trick.
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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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dsa2 min read
Minimum Operations to Reduce X to Zero [Medium] — Complement Window
Find minimum operations to reduce X to zero from either end by finding the longest middle subarray with sum = total-X.
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dsa1 min read
Longest Mountain in Array [Medium] — Two Pass Expand
Find the longest mountain subarray (strictly increasing then decreasing) using two separate forward scans for up and down slopes.
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dsa1 min read
Longest Turbulent Subarray [Medium] — Two Pointer Direction Track
Find the longest turbulent subarray where comparisons alternate between > and < using two-pointer direction tracking.
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dsa1 min read
Subarrays with K Different Integers [Hard] — atMost(K)−atMost(K−1)
Count subarrays with exactly K distinct integers using the mathematical trick: exactly(K) = atMost(K) - atMost(K-1).
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dsa1 min read
Replace the Substring for Balanced String [Medium] — Two Ptr
Find the minimum length substring to replace so that a string of QWER has equal frequencies, using a shrinkable two-pointer approach.
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dsa2 min read
Min Flips for Alternating Binary String [Medium] — Circular Window
Find minimum character flips to make a binary string alternating by applying a fixed circular sliding window of size n.
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dsa1 min read
Maximum Number of Vowels in a Substring of Given Length [Medium]
Find the maximum number of vowels in any substring of length k using a fixed sliding window with vowel count.
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dsa1 min read
Minimum Swaps to Group All 1s Together II [Medium] — Fixed Window
Find minimum swaps to group all 1s together in a circular binary array using a fixed-size sliding window equal to the count of 1s.
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dsa1 min read
Take K of Each Character From Left and Right [Medium] — Middle Window
Find minimum minutes to collect at least k of each character from ends by finding the longest middle window that can be excluded.
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dsa1 min read
Grumpy Bookstore Owner [Medium] — Fixed Window Bonus
Maximize customer satisfaction by finding the best k-minute window for the bookstore owner to stay non-grumpy.
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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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dsa1 min read
Get Equal Substrings Within Budget [Medium] — Variable Window Cost
Find the maximum length substring you can transform from s to t within a given cost budget using a sliding window on character change costs.
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dsa1 min read
Number of Substrings Containing All Three Characters [Medium]
Count substrings containing at least one a, b, and c using the last-seen indices shortcut for O(n) time.
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dsa1 min read
Subarray Sum Divisible by K [Medium] — Prefix Sum Modulo
Count subarrays with sum divisible by k using prefix sum modulo k and a frequency map of remainders.
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dsa1 min read
Find K Closest Elements [Medium] — Binary Search + Two Ptr
Find k closest integers to x in a sorted array using binary search to locate the optimal left boundary of the window.
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dsa1 min read
Count Subarrays With Fixed Bounds [Hard] — One-Pass Window
Count subarrays where min=minK and max=maxK using a single pass tracking the last invalid position and last positions of minK and maxK.
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dsa1 min read
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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dsa1 min read
Sort Vowels in a String [Medium] — Extraction and Replace
Sort only the vowels in a string while keeping consonants in place by extracting vowels, sorting, then reinserting.
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dsa2 min read
Minimum Window Subsequence [Hard] — Forward + Backward Two Ptr
Find the minimum window in s that contains t as a subsequence by using a forward pass to find a valid window then backward pass to minimize it.
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dsa2 min read
Count Subarrays Where Median Equals K [Hard] — Prefix Balance
Count subarrays where the median is exactly k by converting the problem to counting balanced prefix sequences around k.
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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.
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dsa2 min read
Substring with Concatenation of All Words [Hard] — Multi-Start Window
Find all starting indices of substrings that are concatenations of all given words using a sliding window for each possible word-aligned start.
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dsa1 min read
Minimum Consecutive Cards to Pick Up [Medium] — HashMap Window
Find the minimum number of consecutive cards that contain a matching pair using a HashMap tracking last seen positions.
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dsa1 min read
Count Subarrays with Score Less Than K [Hard] — Shrinkable Window
Count subarrays where score = sum × length is less than k using a shrinkable sliding window with running sum.
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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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dsa1 min read
Sliding Window Maximum [Hard] — Monotonic Decreasing Deque
Find the maximum in every sliding window of size k in O(n) using a monotonic decreasing deque of indices.
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dsa1 min read
Shortest Subarray with Sum ≥ K [Hard] — Monotonic Deque on Prefix
Find the shortest subarray with sum >= k, handling negative numbers correctly using a monotonic deque on prefix sums.
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dsa1 min read
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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dsa1 min read
Longest Subarray of Ones After Deleting One Element [Medium]
Find the longest contiguous subarray of 1s after deleting exactly one element using a sliding window allowing at most one zero.
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dsa1 min read
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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dsa1 min read
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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dsa1 min read
Partition Array Into 3 Parts With Equal Sum [Easy] — Three-Pointer
Check if an array can be split into three consecutive parts with equal sum using a greedy counting approach.
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dsa1 min read
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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dsa4 min read
Two Pointers & Sliding Window — Master Cheatsheet (Problems 101–160)
Complete cheatsheet of Two Pointers and Sliding Window patterns with template code, problem index, and MAANG priority list.
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dsa2 min read
Google — Sliding Window Maximum (Monotonic Deque)
Find the maximum in every sliding window of size k using a monotonic decreasing deque. O(n) solution that Google uses to test understanding of amortised data structures.
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dsa2 min read
Google — Minimum Window Substring (Sliding Window + Frequency Map)
Find the smallest substring of s containing all characters of t using an expanding/contracting two-pointer window with frequency counting. O(n) time.
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dsa2 min read
Google — Longest Substring with K Distinct Characters
Find the longest substring with at most k distinct characters using a sliding window with a character frequency map. O(n) time.
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dsa3 min read
Design Hit Counter — Count Requests in Last 300 Seconds
Design a hit counter that counts hits in the last 5 minutes using a deque-based sliding window or circular buffer with O(1) amortised operations.
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