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37 articles
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.
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.
Move all zeroes to end while maintaining relative order of non-zero elements. Optimal O(n) two-pointer solution with minimal writes. Full C, C++, Java, JavaScript and Python code.
Merge two sorted arrays in-place by filling from the end. The classic 3-pointer technique asked at Meta and Microsoft. Full solutions in C, C++, Java, JavaScript and Python.
Find the element that appears once while every other appears twice. The XOR bit trick gives O(n) time and O(1) space. Full solutions in C, C++, Java, JavaScript, Python plus Single Number II and III extensions.
Reverse a character array in-place using two pointers. O(n) time, O(1) space. The fundamental two-pointer swap pattern.
Determine if a string is a palindrome considering only alphanumeric characters and ignoring cases. Two pointer O(n) O(1) solution with all 5 language implementations.
Find the missing number in [0,n]. Two elegant O(n) O(1) solutions: Gauss formula and XOR trick. Full solutions in C, C++, Java, JavaScript and Python.
Find all integers in [1,n] missing from array of length n using O(n) time O(1) extra space via index negation.
Find maximum consecutive ones if you can flip at most k zeros to one. Variable sliding window O(n) solution.
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.
Determine if you can reach the last index. Greedy: track maximum reachable index. If current position exceeds max reach, return false. O(n) O(1).
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.
Check if a string is a palindrome after removing non-alphanumeric characters using inward two pointers.
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.
Meta's interview process: core values evaluation, coding speed expectations, and preparation strategies for Meta's unique product-focused behavioral 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.
Implement an iterator for a nested list that flattens it lazily. Meta favors this for testing iterator design, stack usage, and lazy evaluation.
Serialize a binary tree to a string and deserialize it back. Meta loves this problem for testing tree traversal and string parsing. Both BFS and preorder DFS approaches shown.
Implement a read() function using a read4() primitive. Two variants: read once and read multiple times. Tests buffer management and pointer arithmetic.
Generate all combinations of well-formed parentheses of given n pairs using backtracking. Track open and close counts to prune invalid branches early.
Count subarrays with sum equal to k using prefix sums and a hashmap. O(n) time by tracking how many times each prefix sum has occurred.
Return the values visible from the right side of a binary tree — the last node of each BFS level. O(n) BFS with level tracking.
Merge accounts sharing common email addresses using Union-Find. Group emails by owner, merge accounts with shared emails, sort each merged group.
Create a deep copy of an undirected graph using DFS with a HashMap to track visited/cloned nodes. Meta tests this to evaluate graph traversal and pointer manipulation.
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.