dsa1 min read
Repeated String Match [Medium] — Minimum Repetitions
Find the minimum times A must repeat so that B is a substring, using a tight bound on the number of repeats needed.
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dsa17 min read
Trapping Rain Water [Hard] — Build the Intuition From Scratch (LeetCode 42)
LeetCode 42 is a benchmark Hard problem used by Amazon, Google, and Meta to test whether you can derive an insight — not just memorize code. Learn all three approaches (brute force, prefix arrays, two pointers) and — crucially — understand WHY the two-pointer invariant works.
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dsa15 min read
Sliding Window Maximum [Hard] — Monotonic Deque Explained (LeetCode 239)
Most candidates brute-force this in O(n·k) and hit TLE. Learn the monotonic deque invariant that reduces it to O(n) — with a step-by-step dry run, the five common bugs that break implementations, and why this pattern unlocks Jump Game VI and Constrained Subsequence Sum.
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dsa17 min read
First Missing Positive [Hard] — The Definitive Index-Marking Guide [Amazon, Google, Microsoft]
LeetCode 41 is a landmark Hard problem because both obvious approaches — hash set and sorting — are explicitly banned by the constraints. Learn the mathematical insight that bounds the answer to [1, n+1], then master two O(n) time, O(1) space techniques: index marking via sign negation and cyclic sort placement, with a full visual dry run, bug catalogue, and FAANG follow-ups.
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dsa17 min read
Largest Rectangle in Histogram [Hard] — Monotonic Stack Deep Dive [Amazon, Google]
Master LeetCode 84 the right way. Learn exactly why bars are popped from the monotonic stack, how to calculate left/right boundaries without bugs, the sentinel-values trick that eliminates edge cases, and a step-by-step dry run on [2,1,5,6,2,3] — plus how this pattern extends directly to Maximal Rectangle (LC 85).
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