Divide-and-Conquer

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 first

Merge sort templates, where to count in inversion-style problems, the linked list version, and common mistakes

LeetCode 23 note: a heap that holds only k heads, and divide-and-conquer pairwise merging

LeetCode 493 note: when the counting condition differs from the sort order, scan first, then merge

LeetCode 2426 note: simplify the inequality, then decide whether the matches form a prefix or a suffix

LCR 170 note: count inversions during the merge step

LeetCode 912 note: hand-written merge sort and the two bugs I hit

LeetCode 315 note: merge sort with indices, plus a BIT solution

LeetCode 327 note: prefix sums plus merge sort counting with two pointers bounding a range, and a log of retyping it over a dozen times

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 slidable

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 ties

LeetCode 109 note: take the middle found with fast and slow pointers as the root and recurse on both halves; the height balances naturally