Arrays Hard, Strings, OA Diagnostic #1, Data Science Toolkit, Phase 2 Gate

3 Sum, 4 Sum, Merge Overlapping Intervals, Count Inversions (modified merge sort). TUF+ A2Z Step 3.3

Reverse Words in a String, Valid Anagram, Isomorphic Strings, Longest Common Prefix, Roman to Integer, Implement Atoi, Count Substrings with K Distinct Chars, Longest Palindromic Substring. TUF+ A2Z Step 5.1-5.2

Sit 2 unseen Arrays & Strings problems you have never attempted (pick from the LeetCode topic tag, NeetCode “similar questions” or a company list; never a problem named in any learning task), at least 72 h after the learning task — move the set to the first half of next week if needed. Format: 25 min per problem (35 min for a Hard), including 5 min for edge cases and a dry run; no hints, no AI. Pass bar: ≥ 1/2 solved inside the time box; on a miss, log the miss reason in the Journal and re-sit a fresh pair within 7 days. Ledger checkpoint: ≥ 50 unique problems logged in the Journal (cumulative; tracked metric, Architecture §3).

Count Subarrays with Sum K, Longest Subarray with 0 Sum. TUF+ A2Z Step 3.2

Breadth pass (Pass 1) — Contains Duplicate (LC #217), Encode and Decode Strings (LC #271; LintCode 659), Group Anagrams (LC #49), Top K Frequent Elements (LC #347), Product of Array Except Self (LC #238), Valid Sudoku (LC #36). NeetCode 150

Sit a 90-minute HackerRank/Codility-style OA (2 coding problems + 10 MCQs) under proctor rules (no AI, no hints, single tab). Log every question, your approach and the miss reason in the Journal. Includes a 60-minute post-mortem (the exam alone is 90 minutes).

On HackerRank, Codility and CodeSignal practice pages: Java stdin/stdout parsing, required class naming (Main/Solution), custom test cases, tab-switch and copy-paste rules. Submit one trivial problem on each.

Range Sum Query 2D (LC #304), Corporate Flight Bookings (LC #1109, difference array), Subarray Sums Divisible by K (LC #974). The top OA pattern beyond 1-D prefix sums.

Vectorized operations, array slicing, broadcasting, DataFrame filtering, groupby, merge/join, handling missing data. Complete 30+ exercises. Architecture §6.2

Calendar check against the Opportunity Calendar (Architecture §16); send ≥ 3 personalised outreach messages (alumni, seniors, recruiters) and log each in the Tracker; follow up open threads; ask for a referral 1–2 days after any OA invite; apply within 72 h of any new posting on the Target-40.

Baseline test: 25-minute timed mixed set (quant, logical reasoning, data interpretation) from IndiaBix / PrepInsta-style banks; log accuracy and seconds per question in the Tracker. Target: ≥ 70% accuracy by W28. If the baseline is below 60%, the W11–W12 Aptitude Teaching Blocks become MUST (Architecture §3).