Merge-Sort ·

LeetCode 4099: Minimum Operations to Sort With Locked Indices

2026-11-02·3 mins

LeetCode Medium Array Merge-Sort Greedy

LeetCode 4099 note: the swaps needed equal the inversion count; every locked position some element must cross has to be unlocked, so count those, compare with k, and add the inversions counted with merge sort

Merge Sort and Divide-and-Conquer Counting

2026-10-20·7 mins

Algorithm Merge-Sort Divide-and-Conquer Sorting Counting

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

LeetCode 23: Merge k Sorted Lists

2026-10-20·4 mins

LeetCode Hard Linked-List Heap Merge-Sort Divide-and-Conquer

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

LeetCode 493: Reverse Pairs

2026-10-19·4 mins

LeetCode Hard Merge-Sort Divide-and-Conquer Two-Pointers Counting

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

LeetCode 4051: Count Subarrays with Distant Sums

2026-10-19·5 mins

LeetCode Hard Merge-Sort Prefix-Sum Binary-Indexed-Tree Biweekly-Contest

LeetCode 4051 note: prefix sums, complement counting, range counting, and the k = 0 edge case

LeetCode 2426: Number of Pairs Satisfying Inequality

2026-10-19·3 mins

LeetCode Hard Merge-Sort Divide-and-Conquer Two-Pointers Counting

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

LeetCode LCR 170: Count Inversions (Sword Offer 51)

2026-10-18·3 mins

LeetCode Hard Merge-Sort Divide-and-Conquer Counting

LCR 170 note: count inversions during the merge step

LeetCode 912: Sort an Array

2026-10-18·3 mins

LeetCode Medium Sorting Merge-Sort Divide-and-Conquer

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

LeetCode 88: Merge Sorted Array

2026-10-18·3 mins

LeetCode Easy Two-Pointers Merge-Sort

LeetCode 88 note: merge from the back with O(1) extra space

LeetCode 315: Count of Smaller Numbers After Self

2026-10-18·4 mins

LeetCode Hard Merge-Sort Divide-and-Conquer Binary-Indexed-Tree Counting

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

LeetCode 21: Merge Two Sorted Lists

2026-10-18·3 mins

LeetCode Easy Linked-List Two-Pointers Merge-Sort

LeetCode 21 note: the dummy node, and why Python assignment is not a copy

LeetCode 327: Count of Range Sum

2026-08-01·3 mins

LeetCode Hard Merge-Sort Divide-and-Conquer Prefix-Sum Binary-Indexed-Tree

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