https://compoundengineer.com Visual, step-by-step explanations of DSA, system design, AI and ML systems, DevOps and career roadmaps — built so hard ideas stick before your next interview. en https://compoundengineer.com/library/ai-system-design/design-a-real-time-spam-and-fraud-detection-system https://compoundengineer.com/library/ai-system-design/design-a-real-time-spam-and-fraud-detection-system Tue, 27 Oct 2026 00:00:00 GMT Case studies How to defend a real-time spam and fraud detector in an interview, from baseline and capacity math to labels, failures and evaluation. https://compoundengineer.com/library/dsa/largest-rectangle-in-histogram https://compoundengineer.com/library/dsa/largest-rectangle-in-histogram Tue, 27 Oct 2026 00:00:00 GMT Stacks Find the biggest rectangle in a bar chart. Work out the nearest shorter bar on each side of every bar, then find them all in one pass with a stack. https://compoundengineer.com/library/dsa/container-with-most-water https://compoundengineer.com/library/dsa/container-with-most-water Tue, 27 Oct 2026 00:00:00 GMT Sliding Window Choose two vertical lines that hold the most water. Start with the widest pair and always move the shorter line inward, which turns an O(n²) search into one pass. https://compoundengineer.com/library/dsa/monotonic-stack https://compoundengineer.com/library/dsa/monotonic-stack Tue, 27 Oct 2026 00:00:00 GMT Concepts A stack that keeps its values in order lets every element find its next smaller (or larger) neighbour in one pass. How it works, why it is O(n), and where it shows up. https://compoundengineer.com/library/dsa/depth-first-search https://compoundengineer.com/library/dsa/depth-first-search Mon, 26 Oct 2026 00:00:00 GMT Concepts Explore everything reachable from a start by going as deep as possible before backing up. The visited set, the call stack, and where depth-first search is the right tool. https://compoundengineer.com/library/dsa/union-find https://compoundengineer.com/library/dsa/union-find Mon, 26 Oct 2026 00:00:00 GMT Concepts Keep track of which items belong together as links arrive, in almost constant time. Parent pointers, union by size, path halving, and why it beats searching again after every link. https://compoundengineer.com/library/dsa/critical-connections-in-a-network https://compoundengineer.com/library/dsa/critical-connections-in-a-network Mon, 26 Oct 2026 00:00:00 GMT Graphs & Union-Find Find every link whose loss cuts the network in two. Why "remove each link and search again" is too slow, and how one DFS with discovery times finds all the bridges. https://compoundengineer.com/library/dsa/dijkstras-algorithm https://compoundengineer.com/library/dsa/dijkstras-algorithm Mon, 26 Oct 2026 00:00:00 GMT Concepts Shortest paths in a weighted network, one node at a time. Why the closest unfinished node is always final, how a heap finds it, and why a negative edge breaks the idea. https://compoundengineer.com/library/dsa/bridges-and-low-link https://compoundengineer.com/library/dsa/bridges-and-low-link Mon, 26 Oct 2026 00:00:00 GMT Concepts Find every link whose loss splits a network, in one DFS. Discovery times, low-link values, and the one comparison that decides whether an edge is a bridge. https://compoundengineer.com/library/dsa/combination-sum https://compoundengineer.com/library/dsa/combination-sum Sun, 25 Oct 2026 00:00:00 GMT Recursion & Backtracking Find every way to hit a target by reusing numbers. Why passing a start index to the recursion removes duplicate combinations without any set. https://compoundengineer.com/library/code-system-design/design-a-parking-lot https://compoundengineer.com/library/code-system-design/design-a-parking-lot Sun, 25 Oct 2026 00:00:00 GMT Classic design problems Design a parking lot the way an interview goes. Ask the questions, find the classes, then write the code and handle two cars arriving at once. https://compoundengineer.com/library/dsa/k-closest-points-to-origin https://compoundengineer.com/library/dsa/k-closest-points-to-origin Sun, 25 Oct 2026 00:00:00 GMT Heaps & Priority Queues Return the k points nearest the origin. Why a max-heap of size k beats sorting everything, and why the heap's top is always the point to throw out. https://compoundengineer.com/library/dsa/longest-substring-without-repeating-characters https://compoundengineer.com/library/dsa/longest-substring-without-repeating-characters Sun, 25 Oct 2026 00:00:00 GMT Sliding Window Find the longest stretch of a string with no repeated letter. Grow a window to the right, and when a letter repeats, shrink it from the left until the repeat is gone. https://compoundengineer.com/library/dsa/number-of-islands https://compoundengineer.com/library/dsa/number-of-islands Sun, 25 Oct 2026 00:00:00 GMT Graphs & Union-Find Count the groups of connected land in a grid. Why the first land cell you meet starts a new island, and how a BFS flood fill that sinks cells as it queues them counts each island exactly once. https://compoundengineer.com/library/dsa/coin-change https://compoundengineer.com/library/dsa/coin-change Sun, 25 Oct 2026 00:00:00 GMT Dynamic Programming Find the fewest coins that add up to an amount. Build the answer for every smaller amount first, because the best way to make a is one coin plus the best way to make what is left. https://compoundengineer.com/library/dsa/course-schedule-ii https://compoundengineer.com/library/dsa/course-schedule-ii Sun, 25 Oct 2026 00:00:00 GMT Graphs & Union-Find Order courses so every prerequisite comes first, or report that no order exists. Why a course with zero unmet prerequisites is always safe to take next, and how a leftover course reveals a cycle. https://compoundengineer.com/library/dsa/task-scheduler https://compoundengineer.com/library/dsa/task-scheduler Sun, 25 Oct 2026 00:00:00 GMT Heaps & Priority Queues Run all tasks with a cooldown between two runs of the same task, in the fewest time slots. Why always running the task with the most copies left is optimal, simulated with a max-heap and a cooldown queue. https://compoundengineer.com/library/dsa/word-search https://compoundengineer.com/library/dsa/word-search Sun, 25 Oct 2026 00:00:00 GMT Recursion & Backtracking Find a word by walking through a grid of letters. Why the search marks each cell as visited on the way in and must restore it on the way out. https://compoundengineer.com/library/dsa/longest-increasing-subsequence https://compoundengineer.com/library/dsa/longest-increasing-subsequence Sun, 25 Oct 2026 00:00:00 GMT Dynamic Programming Find the length of the longest strictly increasing subsequence. Keep the smallest possible tail for every length, and binary search that sorted list to place each number. https://compoundengineer.com/library/dsa/merge-k-sorted-lists https://compoundengineer.com/library/dsa/merge-k-sorted-lists Sun, 25 Oct 2026 00:00:00 GMT Heaps & Priority Queues Merge k sorted linked lists into one sorted list. Why the next node of the answer is always one of the k list heads, and how a min-heap picks it in O(log k). https://compoundengineer.com/library/dsa/minimum-window-substring https://compoundengineer.com/library/dsa/minimum-window-substring Sun, 25 Oct 2026 00:00:00 GMT Sliding Window Find the shortest part of a string that contains every letter of another string. Grow a window until it is valid, then shrink it from the left for as long as it stays valid. https://compoundengineer.com/library/dsa/n-queens https://compoundengineer.com/library/dsa/n-queens Sun, 25 Oct 2026 00:00:00 GMT Recursion & Backtracking Place n queens on an n by n board so none attack each other. Why one queen per row and three small sets let the search drop a bad board the moment it appears. https://compoundengineer.com/library/dsa/find-median-from-data-stream https://compoundengineer.com/library/dsa/find-median-from-data-stream Sun, 25 Oct 2026 00:00:00 GMT Heaps & Priority Queues Keep the median of a growing stream available in O(1). Split the numbers into a lower half in a max-heap and an upper half in a min-heap, and the median is always sitting on the tops. https://compoundengineer.com/library/dsa/word-ladder https://compoundengineer.com/library/dsa/word-ladder Sun, 25 Oct 2026 00:00:00 GMT Graphs & Union-Find Turn one word into another by changing a single letter at a time, using only words from a list. Why breadth-first search gives the shortest ladder, and why each word must be marked visited the moment it is queued. https://compoundengineer.com/library/dsa/edit-distance https://compoundengineer.com/library/dsa/edit-distance Sun, 25 Oct 2026 00:00:00 GMT Dynamic Programming Find the fewest insert, delete and replace operations that turn one word into another. Fill a table where each cell is the best cost for a pair of prefixes, built from its left, top and diagonal neighbours. https://compoundengineer.com/library/dsa/swim-in-rising-water https://compoundengineer.com/library/dsa/swim-in-rising-water Sun, 25 Oct 2026 00:00:00 GMT Graphs & Union-Find Cross a grid while the water rises, so a path costs its tallest cell. Why a priority queue that always steps onto the lowest reachable cell finds the earliest moment you can swim across, and how Union-Find gets there another way. https://compoundengineer.com/library/dsa/maximal-rectangle https://compoundengineer.com/library/dsa/maximal-rectangle Sun, 25 Oct 2026 00:00:00 GMT Dynamic Programming Find the largest rectangle of 1s in a binary matrix. Turn each row into a histogram of consecutive 1s above it, then solve Largest Rectangle in Histogram with a monotonic stack. https://compoundengineer.com/library/dsa/house-robber https://compoundengineer.com/library/dsa/house-robber Sat, 24 Oct 2026 00:00:00 GMT Dynamic Programming Rob the most money from a row of houses without hitting two neighbours. Build the best total house by house, where each house either adds its loot to the best from two houses back, or is skipped. https://compoundengineer.com/library/dsa/maximum-product-subarray https://compoundengineer.com/library/dsa/maximum-product-subarray Sat, 24 Oct 2026 00:00:00 GMT Dynamic Programming Find the contiguous subarray with the largest product. A negative number can turn the smallest product into the largest, so track both the maximum and the minimum product ending at each index. https://compoundengineer.com/library/dsa/sliding-window-maximum https://compoundengineer.com/library/dsa/sliding-window-maximum Sat, 24 Oct 2026 00:00:00 GMT Sliding Window Find the maximum of every window of size k in one pass. Keep a deque of indices whose values only decrease, and each element is added and removed at most once.