https://wid-blog.github.io/en/posts/tech/algorithm/Recent content in Algorithm on wid's blogHugoenThu, 09 Jul 2026 00:00:00 +0000https://wid-blog.github.io/en/posts/tech/algorithm/top-k-heap-vs-quickselect/Thu, 09 Jul 2026 00:00:00 +0000https://wid-blog.github.io/en/posts/tech/algorithm/top-k-heap-vs-quickselect/A heap is the reflex answer to K-th largest problems, but when the whole array is already in memory, Quick Select is faster on average. The size-K heap and Quick Select mechanics, and how the shape of the data decides between them.https://wid-blog.github.io/en/posts/tech/algorithm/union-find/Sat, 04 Jul 2026 00:00:00 +0000https://wid-blog.github.io/en/posts/tech/algorithm/union-find/Union-Find gives each group a root representative and decides whether two elements share a group. The unoptimized version stretches find to O(N) on chain trees; path compression and union by rank make it practically constant.https://wid-blog.github.io/en/posts/tech/algorithm/topological-sort/Fri, 03 Jul 2026 00:00:00 +0000https://wid-blog.github.io/en/posts/tech/algorithm/topological-sort/Kahn’s BFS for topological sort repeatedly takes nodes with zero in-degree, and cycle detection comes along without extra logic. The mechanism, the Course Schedule problems, and the selection criteria against DFS post-order.https://wid-blog.github.io/en/posts/tech/algorithm/sliding-window-and-prefix-sum/Sun, 14 Jun 2026 00:00:00 +0000https://wid-blog.github.io/en/posts/tech/algorithm/sliding-window-and-prefix-sum/Subarray-sum problems collapse from O(N²) to O(N) along two paths — sliding window or prefix sum with a hash. Negative inputs or strict-equality conditions break monotonicity and shift the work from sliding window to prefix sum.https://wid-blog.github.io/en/posts/tech/algorithm/parametric-search/Sat, 13 Jun 2026 00:00:00 +0000https://wid-blog.github.io/en/posts/tech/algorithm/parametric-search/When the answer isn’t sitting in any array, you can still binary-search the answer itself. The parametric-search recipe — define the decision function, fix the direction of monotonicity, justify lo and hi.