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Two Sum is the most asked interview problem at Google, Amazon and Meta. Learn every approach from O(n²) brute force to O(n) HashMap with C, C++, Java, JavaScript and Python solutions.
The classic stock problem asked at Amazon, Google and Microsoft. Learn the O(n) greedy "min so far" approach with complete solutions in C, C++, Java, JavaScript and Python, plus extensions to Stock II, III and IV.
Kadane's Algorithm is a must-know pattern for FAANG interviews. Solve Maximum Subarray in O(n) time with full solutions in C, C++, Java, JavaScript and Python, plus divide-and-conquer O(n log n) alternative.
Increment a large integer represented as an array by one. Learn the carry propagation pattern with O(n) solutions in C, C++, Java, JavaScript and Python.
Find the longest common prefix string among an array of strings. Covers vertical scan O(n*m), horizontal scan, binary search, and Trie approaches with all 5 language solutions.
Find the majority element that appears more than n/2 times. Boyer-Moore Voting Algorithm achieves O(n) time and O(1) space. Full C, C++, Java, JavaScript and Python solutions.
Find maximum consecutive ones if you can flip at most k zeros to one. Variable sliding window O(n) solution.
Implement Fisher-Yates shuffle for uniform random permutation. Every arrangement is equally probable. O(n) time O(1) extra space.
Find all unique triplets summing to zero. Sort array then fix one element and use two pointers. Full O(n²) solution in C, C++, Java, JavaScript and Python.
Find two lines that together with the x-axis form a container holding the most water. Greedy two-pointer O(n) — never check a pair that can't be optimal.
Compute product of all elements except self without division. O(n) two-pass prefix/suffix approach with O(1) extra space.
Merge all overlapping intervals. Sort by start, then merge greedily in one pass. O(n log n) time.
Return all elements of an m×n matrix in spiral order. Use boundary shrinking or direction array. O(m*n) time O(1) space.
Group strings that are anagrams of each other. Use sorted string as HashMap key. O(n*k*log k) time. Full solutions in 5 languages.
Find length of longest substring without repeating characters. Optimal O(n) sliding window with HashMap tracking last seen positions.
Count subarrays summing to k. Classic prefix sum + HashMap trick: if prefix[j]-prefix[i]=k then prefix[i]=prefix[j]-k. O(n) time.
Search a target in a rotated sorted array with no duplicates. Modified binary search identifies which half is sorted, then checks the target range. O(log n) time.
Find the minimum element in a rotated sorted array with no duplicates. Binary search: the minimum is in the unsorted half. O(log n) time O(1) space.
Find k most frequent elements. Approach 1: min-heap O(n log k). Approach 2: bucket sort O(n). Full 5-language solutions.
Find the maximum product of a contiguous subarray. Track both min and max at each position (negative * negative = positive). O(n) time O(1) space.
Rotate an n×n matrix 90 degrees clockwise in-place. Trick: transpose (swap i,j with j,i) then reverse each row. O(n²) time O(1) space.
Find the lexicographically next permutation in-place using a two-pass scan from the right.
Find the one duplicate in [1..n] without modifying the array using Floyd's tortoise-and-hare cycle detection.
Insert a new interval into a sorted non-overlapping list and merge any overlaps in a single sweep.
Find the minimum number of intervals to remove so the rest are non-overlapping, using a greedy earliest-end-time strategy.
Decode a run-length-encoded string like "3[a2[bc]]" = "abcbcabcbcabcbc" using a stack for nested brackets.
Determine the unique starting gas station for a circular route using a greedy one-pass algorithm.
Find how many days until a warmer temperature for each day using a monotonic decreasing stack.
Find the longest consecutive integer sequence in O(n) using a HashSet and only extending sequences from their start.
Partition a string into the maximum number of parts where each letter appears in at most one part, using last occurrence mapping.
Find the minimum arrows needed to burst all balloons arranged as intervals using a greedy sort-by-end approach.
Find a peak element in O(log n) by binary searching on the slope direction — always move toward the higher neighbor.
Find all elements appearing more than n/3 times using Extended Boyer-Moore Voting with two candidate trackers.
Rearrange an array so nums[0] < nums[1] > nums[2] < nums[3]... using virtual index mapping and nth_element for O(n) time.
Distribute minimum candies so each child with higher rating than neighbors gets more, using left-to-right then right-to-left passes.
Rearrange nums to maximize advantage over B using a greedy strategy: assign the smallest winning card, else discard the smallest.
Check if string B is a rotation of A by checking if B appears in A+A using a substring search.
Find the minimum times A must repeat so that B is a substring, using a tight bound on the number of repeats needed.
Calculate trapped rain water using inward two pointers that track left-max and right-max, replacing the classic O(n) space approach.
Find the maximum in every sliding window of size k in O(n) using a monotonic decreasing deque.
Find the smallest missing positive integer in O(n) time O(1) space using in-place cyclic sort to place each number at its correct index.
Find the largest rectangle in a histogram in O(n) using a monotonic increasing stack that computes area when a shorter bar is encountered.
Find the minimum window in s that contains all characters of t using a shrinkable sliding window with a character frequency counter.
Count elements smaller than each element to its right using a modified merge sort that counts inversions during the merge step.
Find the median of two sorted arrays in O(log(min(m,n))) by binary searching for the correct partition point.
Find the largest rectangle of 1s in a binary matrix by treating each row as a histogram and applying the Largest Rectangle in Histogram algorithm.
Count pairs (i,j) where i<j and nums[i]>2*nums[j] using modified merge sort to count cross-half pairs before merging.
Minimize the largest sum among m subarrays by binary searching on the answer and greedy checking feasibility.
Find the shortest subarray with sum >= K using a monotonic deque on prefix sums, handling negative numbers correctly.
Count the number of range sums that lie in [lower, upper] using a merge sort approach on prefix sums.
Find three non-overlapping subarrays of length k with maximum sum using sliding window sums and left/right best index arrays.
Find the minimum refueling stops to reach target by greedily picking the largest fuel station passed so far whenever we run out.
Find the longest subarray of 1s after deleting exactly one element using a sliding window that tracks the count of zeros.
Find the maximum number of points on the same line using a slope-as-fraction HashMap for each anchor point.
Find the longest valid parentheses substring using a stack that tracks the last unmatched index as a base.
Find the shortest subarray that when sorted makes the whole array sorted, using a single linear scan tracking violated boundaries.
Count subarrays with exactly K distinct integers using the formula: atMost(K) - atMost(K-1).
Reconstruct the stamp sequence in reverse by greedily finding where the stamp can overwrite current characters in the target string.
Find the longest substring that appears at least twice using binary search on length and Rabin-Karp rolling hash for O(n log n) average time.
Design a stack that pops the most frequent element (breaking ties by recency) using frequency and group-stacks mapping.
Find two indices that add to target in O(n) by storing each number in a HashMap and looking up the complement.
Check if a ransom note can be constructed from magazine letters using a character frequency map.
Check if two strings are isomorphic by maintaining bidirectional character mappings to ensure a consistent one-to-one correspondence.
Check if a string follows a given pattern using bidirectional mapping between pattern chars and words.
Find if any two duplicate elements are within k indices of each other using a HashMap of last-seen positions.
Find characters common to all strings in a list by intersecting their frequency arrays with element-wise minimum.
Find all groups of duplicate files by parsing path strings and grouping files by their content using a HashMap.
Group strings that are anagrams together by using their sorted form as a HashMap key.
Find the k most frequent elements in O(n) using bucket sort by frequency instead of a heap.
Design a Least Recently Used cache with O(1) get and put operations using a HashMap combined with a doubly linked list.
Count subarrays with sum exactly k using prefix sums stored in a HashMap to find valid starting positions in O(n).
Check if a continuous subarray of length >= 2 sums to a multiple of k using prefix sum modulo k with a HashMap.
Find the longest consecutive integer sequence in O(n) by using a HashSet and only starting sequences from their minimum element.
Design a set with O(1) insert, delete, and getRandom using a dynamic array combined with a HashMap for index tracking.
Find all starting indices of anagram substrings by using a fixed window with character frequency comparison.
Implement weighted random selection by building a prefix sum array and using binary search to find the sampled index.
Find the vertical line that crosses the fewest bricks by counting the most frequent gap position using a HashMap.
Reverse an array of characters in-place using the classic two-pointer swap technique.
Return squares of a sorted array in sorted order using two pointers that compare absolute values from both ends.
Merge two sorted arrays in-place from the end to avoid overwriting elements using three pointers.
Check if string s is a subsequence of t using a greedy two-pointer scan that matches characters in order.
Find the maximum number of consecutive 1s achievable by flipping at most k zeros, using a variable sliding window.
Count days with fixed-size window sums above upper and below lower thresholds using a sliding fixed window.
Find the longest substring without duplicate characters using a HashSet-backed shrinkable sliding window.
Find the longest substring with at most k character replacements by tracking the maximum frequency char in the window.
Check if any permutation of s1 is a substring of s2 using a fixed-size sliding window with character frequency comparison.
Find the maximum fruits you can collect starting from any tree with two baskets (at most 2 distinct fruit types) using a sliding window.
Find the maximum sum of a subarray with all unique elements using a sliding window backed by a HashSet.
Find the smallest subarray with sum >= target using a variable sliding window that shrinks from the left when the condition is met.
Count tuples (i,j,k,l) where nums1[i]+nums2[j]+nums3[k]+nums4[l]=0 by hashing all pairs from first two arrays.
Maximize water trapped between two vertical lines using inward two pointers that always move the shorter line inward.
Find minimum boats to save everyone where each boat carries at most 2 people with total weight <= limit, using sort + greedy two pointers.
Find all unique triplets summing to zero by sorting and using two pointers for each fixed element with careful duplicate skipping.
Count contiguous subarrays with product < k using a sliding window that divides from the left when product exceeds the limit.
Maximize card points taken from both ends of an array by finding the minimum-sum subarray of size n-k in the middle.
Count subarrays with exactly k odd numbers using the at-most(k) minus at-most(k-1) sliding window formula.
Count binary subarrays with exactly the given sum using prefix sum with HashMap or the atMost(k)-atMost(k-1) trick.
Find the maximum frequency of any element after at most k increments by sorting and using a sliding window with a running sum.
Find minimum operations to reduce X to zero from either end by finding the longest middle subarray with sum = total-X.
Find the longest mountain subarray (strictly increasing then decreasing) using two separate forward scans for up and down slopes.
Find the longest turbulent subarray where comparisons alternate between > and < using two-pointer direction tracking.
Count subarrays with exactly K distinct integers using the mathematical trick: exactly(K) = atMost(K) - atMost(K-1).
Find the minimum length substring to replace so that a string of QWER has equal frequencies, using a shrinkable two-pointer approach.
Find minimum character flips to make a binary string alternating by applying a fixed circular sliding window of size n.
Find the maximum number of vowels in any substring of length k using a fixed sliding window with vowel count.
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.
Find minimum minutes to collect at least k of each character from ends by finding the longest middle window that can be excluded.
Maximize customer satisfaction by finding the best k-minute window for the bookstore owner to stay non-grumpy.
Find the minimum difference between the max and min of any k scores by sorting and using a fixed window of size k.
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.
Count substrings containing at least one a, b, and c using the last-seen indices shortcut for O(n) time.
Count subarrays with sum divisible by k using prefix sum modulo k and a frequency map of remainders.
Find k closest integers to x in a sorted array using binary search to locate the optimal left boundary of the 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.
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.
Count subarrays where the median is exactly k by converting the problem to counting balanced prefix sequences around k.
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.
Find all starting indices of substrings that are concatenations of all given words using a sliding window for each possible word-aligned start.
Find the minimum number of consecutive cards that contain a matching pair using a HashMap tracking last seen positions.
Count subarrays where score = sum × length is less than k using a shrinkable sliding window with running sum.
Find the smallest window in s containing all characters of t using a have/need counter to track when the window is valid.
Find the maximum in every sliding window of size k in O(n) using a monotonic decreasing deque of indices.
Find the shortest subarray with sum >= k, handling negative numbers correctly using a monotonic deque on prefix sums.
Find the longest substring containing at most 2 distinct characters using a variable sliding window with a character count map.
Find the longest contiguous subarray of 1s after deleting exactly one element using a sliding window allowing at most one zero.
Check if an array can be split into three consecutive parts with equal sum using a greedy counting approach.
Calculate trapped rainwater using optimal two-pointer approach that tracks left and right maximums without extra space.
High-frequency string problems from Meta and Google interviews: valid parentheses, longest substring without repeating characters, zigzag conversion, and string to integer.
Full company-specific mock sessions. Each simulation replicates the actual interview format, problem difficulty distribution, and evaluation criteria of Google, Meta, and Amazon.
Google's interview process: Googleyness criteria, coding expectations, and how to prepare for the unique 'How would you improve Google Maps?' style questions.
Strategic guide to company-specific DSA preparation: Google's focus on graphs and DP, Meta's emphasis on arrays and trees, Amazon's leadership principles alignment with system design and BFS.
Search for a target in a matrix where each row and column is sorted. O(m+n) solution using top-right corner elimination — a classic Google interview problem.
Derive character ordering from a sorted alien dictionary using topological sort. Build a directed graph from adjacent word comparisons and apply Kahn's BFS algorithm.
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.
Find the smallest substring of s containing all characters of t using an expanding/contracting two-pointer window with frequency counting. O(n) time.
Find the longest substring with at most k distinct characters using a sliding window with a character frequency map. O(n) time.
Count the number of inversions in an array (pairs where i < j but arr[i] > arr[j]) using modified merge sort in O(n log n). Google uses this to test deep algorithm understanding.
Return all ways to segment a string into dictionary words. Combines DP validity check with DFS+memo backtracking to avoid TLE on valid inputs.
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.
Complete recap of all 24 company-tagged problems with pattern classification, company attribution, and key insights. Use as a final review checklist before company-specific interviews.