https://h30306.github.io/blog/tags/divide-and-conquer/Recent content in Divide-and-Conquer onHugo -- gohugo.ioenhwork0511@gmail.com (Wei Yi Chung)hwork0511@gmail.com (Wei Yi Chung)© 2026 Wei Yi ChungThu, 29 Oct 2026 00:00:00 +0000https://h30306.github.io/blog/leetcode/leetcode-889/Thu, 29 Oct 2026 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-889/LeetCode 889 note: keep the current path on a stack; when its top equals the next postorder value that subtree is complete and is popped; new nodes go left firsthttps://h30306.github.io/blog/leetcode/leetcode-23/Tue, 20 Oct 2026 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-23/LeetCode 23 note: a heap that holds only k heads, and divide-and-conquer pairwise merginghttps://h30306.github.io/blog/algorithm/merge-sort/Tue, 20 Oct 2026 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/algorithm/merge-sort/Merge sort templates, where to count in inversion-style problems, the linked list version, and common mistakeshttps://h30306.github.io/blog/leetcode/leetcode-2426/Mon, 19 Oct 2026 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-2426/LeetCode 2426 note: simplify the inequality, then decide whether the matches form a prefix or a suffixhttps://h30306.github.io/blog/leetcode/leetcode-493/Mon, 19 Oct 2026 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-493/LeetCode 493 note: when the counting condition differs from the sort order, scan first, then mergehttps://h30306.github.io/blog/leetcode/leetcode-315/Sun, 18 Oct 2026 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-315/LeetCode 315 note: merge sort with indices, plus a BIT solutionhttps://h30306.github.io/blog/leetcode/leetcode-912/Sun, 18 Oct 2026 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-912/LeetCode 912 note: hand-written merge sort and the two bugs I hithttps://h30306.github.io/blog/leetcode/leetcode-lcr-170/Sun, 18 Oct 2026 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-lcr-170/LCR 170 note: count inversions during the merge stephttps://h30306.github.io/blog/leetcode/leetcode-327/Sat, 01 Aug 2026 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-327/LeetCode 327 note: prefix sums plus merge sort counting with two pointers bounding a range, and a log of retyping it over a dozen timeshttps://h30306.github.io/blog/leetcode/leetcode-1763/Sat, 17 Jan 2026 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-1763/LeetCode 1763 note: a character whose other case never appears cannot be in the answer, so split on it and recurse, preferring the left side on tieshttps://h30306.github.io/blog/leetcode/leetcode-395/Sat, 17 Jan 2026 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-395/LeetCode 395 note: “every character at least k times” is not monotonic, so a direct slide fails; divide and conquer splits on characters with fewer than k occurrences, or fixing the number of distinct characters makes the window slidablehttps://h30306.github.io/blog/leetcode/leetcode-109/Sat, 19 Nov 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-109/LeetCode 109 note: take the middle found with fast and slow pointers as the root and recurse on both halves; the height balances naturallyhttps://h30306.github.io/blog/leetcode/leetcode-148/Thu, 17 Nov 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-148/LeetCode 148 note: split in half with fast and slow pointers, sort each half recursively and merge; O(n log n) with no extra arrayhttps://h30306.github.io/blog/leetcode/leetcode-324/Thu, 17 Nov 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-324/LeetCode 324 note: sort, split into a small half and a large half, reverse each, and interleave with small values at even indices and large ones at odd indiceshttps://h30306.github.io/blog/leetcode/leetcode-240/Sat, 12 Nov 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-240/LeetCode 240 note: start at the top-right corner, moving left when larger and down when smaller, eliminating a row or column each stephttps://h30306.github.io/blog/leetcode/leetcode-106/Thu, 10 Nov 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-106/LeetCode 106 note: the last post-order value is the root; taking roots from the end of the post-order, build the right subtree before the lefthttps://h30306.github.io/blog/leetcode/leetcode-105/Sat, 29 Oct 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-105/LeetCode 105 note: the first pre-order value is the root, and its position in the in-order tells the left subtree’s size; look positions up in a dictionary for O(n)https://h30306.github.io/blog/leetcode/leetcode-215/Sat, 29 Oct 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-215/LeetCode 215 note: keep a min-heap of size k whose top is the k-th largest, or use quickselect for O(n) on averagehttps://h30306.github.io/blog/leetcode/leetcode-973/Sat, 29 Oct 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-973/LeetCode 973 note: as in Top-K, keep a size-k max-heap (negated distances) holding the k closest pointshttps://h30306.github.io/blog/leetcode/leetcode-347/Thu, 27 Oct 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-347/LeetCode 347 note: count first, then take the k most frequent with a size-k heap or a bucket sort indexed by counthttps://h30306.github.io/blog/leetcode/leetcode-169/Sat, 20 Aug 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-169/LeetCode 169 note: Boyer-Moore voting, where different values cancel in pairs and the survivor is the one appearing more than half the timehttps://h30306.github.io/blog/leetcode/leetcode-108/Tue, 16 Aug 2022 00:00:00 +0000hwork0511@gmail.com (Wei Yi Chung)https://h30306.github.io/blog/leetcode/leetcode-108/LeetCode 108 note: the middle element becomes the root and each half recurses; the halves differ by at most one, so the tree is balanced