8 articles
Master bottom-up merge sort on a linked list for O(n log n) time and O(1) space — eliminating the O(log n) recursive stack. A hard interview problem at Amazon, Google, and Facebook that demonstrates deep understanding of merge sort and linked list mechanics.
Master LeetCode 315 — one of the most common FAANG hard problems. Learn why a naive O(n²) scan fails at scale, how merge sort secretly counts inversions as a side effect, and how to trace through every swap on paper. Includes both brute-force and optimal solutions in Python and JavaScript, a full complexity table, and the three follow-up problems that frequently appear in the next interview round.
LeetCode 493 trips up even strong candidates because the count step must happen before the merge step — not during it. Learn exactly why that ordering matters, trace through a full dry run on [1,3,2,3,1], understand the five most common bugs, and walk away with clean Python and JavaScript solutions you can reproduce under pressure.
Master LeetCode 327 — Count of Range Sum — with deep intuition, a visual dry run, brute-force to O(n log n) merge sort progression, and real interview follow-ups covering BIT, LC 315, and LC 493.
Count subarrays whose sum lies in [lower, upper] using merge sort on prefix sums in O(n log n). Understand the divide-and-conquer counting trick that makes an O(n^2) brute-force drop to linearithmic time.
LC 148 Sort List is a classic O(n log n) interview problem at Amazon, Google, and Facebook that applies merge sort to a linked list using fast/slow pointer splitting and recursive merging. Learn top-down merge sort with Python and JavaScript, a complete dry run, and the bottom-up O(1) space follow-up.
Count pairs (i, j) with i less than j and nums[i] greater than twice nums[j]. The classic Hard problem for modified merge sort and BIT counting — the two techniques every senior interviewer expects.
Count subarray sums in [lower, upper] using merge sort or Fenwick tree on prefix sums. The flagship Hard problem connecting prefix sums, divide-and-conquer counting, and BIT range queries.