I've been building a service for a month and the day finally arrived when I had the artifact - an EC2 AMI. The AMI passed my "rigourous" manual tests, and I felt confident on a Ruby Tuesday, so I launched 100 EC2 instances using the AMI. Surprise! around 28 instances failed to launch. What is going on? All …

Read More

Given an array of numbers A, find out whether it contains a 1-3-2 pattern. An 1-3-2 pattern is a subsequence of three numbers, A[i], A[j] and A[k] such that i<j<k and A[i] < A[k] < A[j]. For clarity, let's call the 1-3-2 pattern the Bronze-Gold-Silver pattern. If A[j] is Gold, then we should consider the …

Read More

Tony Hoare invented QuickSort in 1961. At the time of its publication, the best comparison-based sorting algorithm was merge sort. Merge sort divides an unordered array into two equally sized subarrays, sorts each subarray, and then merge the two subarrays to produce a sorted array. Merge sort is simple to understand. …

Read More

I often find myself confused by "upstream" and "downstream", in the context of software development. They bother me so much that I avoid using them in my own writing and I have to pause whenever I see them. In this post, I'll show a simple rule that helps you remember the difference: downstream adds …

Read More

Modern CPUs operate significantly faster than memories. A 4.5 GHz x64 CPU operates 30 times faster than a 6000 MHz DDR5 memory of CAS Latency 36. Adding latencies incurred by the bus and memory coherency protocols, memory could be 100 times slower than registers. To mitigate the speed gap, CPU uses layers of caches, …

Read More

Given a non-decreasing array and a target value, we can find the target in logarithmic time using binary search. My first programming language is C++ and the C++ Standard Template Library (STL) provides two functions for this task. iterator lower_bound(first, last, value) returns the smallest index with a value larger …

Read More

Problem Given an integer array A, in one step, remove all elements A[i] where A[i-1] > A[i]. Return the number of steps performed until A becomes a non-decreasing array. See examples at LeetCode 2289. Solution The naive approach executes steps one by one. Store integers in a Linked List. At each step, find all …

Read More

Problem In a connected network consisting of N nodes, each node is connected to either one or two neighbors, forming a line topology. The task is to develop a program that runs on each node, calculating the total number of nodes in the network. Each node is aware of its neighboring nodes and can exchange messages with …

Read More

Problem An event consists of multiple properties, each defined as a key-value pair, where the key is a string and the value is of a primitive type such as numbers or strings. Importantly, each event must include a mandatory 'Name' property. Given a list of events, the task is to count the number of events based on …

Read More

ClickHouse is a popular OLAP database. It speaks SQL and earns the reputation of "fast and resource efficient". But the support of SQL comes with surprises if not careful. In this blog, I show that a simple query of nested WITH clauses in ClickHouse generates factorial number of subqueries. The simple query …

Read More