notes on systems, databases, and competitive programming

2026

2026-06-19 GCDARR2. GCD Arrays (Hard)

CodeChef fix g as the interior GCD, count interior sequences with Möbius inversion, count valid endpoint pairs with inclusion-exclusion — O(M log M + M log N) per test case

2026-06-18 Euler Tour on Trees

DSA Flatten a tree into an array using DFS timestamps — subtree queries become range queries, and you can throw a segment tree at them.

2026-06-12 LCA and Binary Lifting

DSA How to jump up a tree in powers of two — kth ancestors, lowest common ancestors, and path distances, all in O(log n) per query.