12 articles
Learn how to remove consecutive nodes that sum to zero from a linked list using a prefix sum hashmap in two passes. A tricky interview problem asked at Google and Amazon that combines linked list manipulation with the classic prefix sum technique.
Build an LRU Cache from scratch using a doubly linked list with sentinel nodes and a HashMap for O(1) get and put operations. The most frequently asked hard design problem at Amazon, Microsoft, Google, and Facebook — explained step by step with diagrams.
Master LeetCode 76 — the gold-standard Hard sliding window problem asked at Google, Meta, and Amazon. Learn the "formed" counter trick that reduces window validity checks from O(|t|) to O(1), trace through a full dry run, and avoid the five bugs that most often break this one.
LeetCode 149 asks you to find the maximum number of collinear points on a 2D plane. The trick is representing slope as a GCD-reduced integer fraction — no floats, no precision bugs — and using a HashMap to count how many points share the same slope relative to each anchor. This post covers the full intuition, a step-by-step visual dry run, every edge case (vertical lines, duplicates, sign normalization), well-commented Python and JavaScript solutions, and the real FAANG follow-up questions that separate good candidates from great ones.
LC 992 is one of the cleanest examples of a non-obvious reduction in competitive programming. Learn the "exactly K = atMost(K) minus atMost(K-1)" insight, trace through a full visual dry run, and understand how this single pattern unlocks five related hard problems in one shot.
Complete cheatsheet for Hashing & Maps: all 7 patterns, Big O reference, template code, and MAANG priority guide.
LC 138 Copy List with Random Pointer is a medium interview problem at Amazon, Microsoft, and Facebook that requires deep-copying a linked list where each node has a random pointer. Learn the O(n) space hash map approach and the clever O(1) space interleave technique with Python and JavaScript solutions and detailed dry run.
Find the next greater element for each query using a monotonic stack on nums2 and a hash map lookup in O(n+m) time. A classic application of the monotonic stack pattern combined with hash map lookups for efficient query answering.
LeetCode 454 — count quadruples summing to zero across four arrays using meet-in-the-middle hashing. Asked at Google, Amazon, and Bloomberg.
LC 974 counts subarrays whose sum is divisible by k using prefix sums modulo k and a frequency map of remainders. O(n) time, O(k) space. The standard FAANG pattern for all modular subarray problems.
LC 2260 asks for the shortest consecutive sequence of cards containing a matching pair. Track the last-seen index of each card value — update the minimum window each time a duplicate is encountered. O(n) time, O(n) space.
Find the longest substring containing at most 2 distinct characters using a variable sliding window backed by a character frequency map — a premium LinkedIn and Google problem with a clean generalization to k distinct.