6-Month Serious Track — Implementation

This is the execution layer for schedules/6_MONTH_SERIOUS.md. The schedule tells you what to study and when. This folder is the actual substance — curated flagship problems, original company-sourced problems, graduated reading, proof-first solutions, and senior/staff/principal-level signal layers.

What This Is (And Is Not)

This IS:

91 hand-picked flagship problems across 20 active weeks
8–10 ORIGINAL problems sourced from real Amazon/AWS, Google, Meta, Microsoft interview intelligence (not on LeetCode)
A graduated 15-section reading layout per problem: approach → hints → insight → company adversarial → level delta → follow-ups
Python solutions with brute-force comparators and stress tests
Above-FAANG difficulty content targeting senior, staff, and principal interviews

This is NOT:

A duplicate of LeetCode Premium. You should have LC Premium. We never restate problem statements or constraints — those are one click away.
Every problem from the 6-month schedule (~595). The other ~500 are listed as “Problem Bank” in each week’s README — work them on LC after you own the flagship.
A passive read. Each problem has an Attempt Gate — you set a timer and try the problem cold before reading anything else.

Why This Exists

LeetCode editorials give you code. They do not give you:

The exchange argument that proves the greedy works
The invariant that the two-pointer maintains
The misleading example a Google interviewer will use to trap you on this exact problem
The scorecard language the interviewer writes when you do (or do not) ace it
The Level Delta: what a Mid vs Senior vs Staff vs Principal answer looks like for this same problem
The production reality: if this ran at 10M RPS, what would break first?
The Anti-pattern: the wrong-but-tempting approach that catches 70% of candidates, with exact bug location
The original problem Amazon will ask you that has never appeared on LeetCode

This track provides all of those.

How To Use

Read HOW_TO_USE.md — the graduated reading protocol
Read FRAMEWORK.md once if you have not — every “How to Approach” section here references its 16 steps
Start at month-01-foundations/, week 01, problem 01. Do not skip ahead.
For each problem: Attempt Gate first (20-min timer, cold), then graduate through the README sections
Stress-test your solution: python solution.py runs the brute-vs-optimal comparator
Log every solve in your tracking spreadsheet per SPACED_REPETITION.md

Problem Index (All 91)

Month 1 — Foundations (20 problems)

#	Problem	Difficulty	Source	Week
p01	Two Sum	Easy	LC 1	1
p02	Valid Parentheses	Easy	LC 20	1
p03	Best Time to Buy and Sell Stock	Easy	LC 121	1
p04	Merge Sorted Array	Easy	LC 88	1
p05	Climbing Stairs	Easy	LC 70	1
p06	Product of Array Except Self	Medium	LC 238	2
p07	Group Anagrams	Medium	LC 49	2
p08	Rotate Array	Medium	LC 189	2
p09	Longest Substring Without Repeating	Medium	LC 3	2
p10	Valid Anagram	Easy	LC 242	2
p11	Binary Tree Level Order Traversal	Medium	LC 102	3
p12	Kth Largest Element	Medium	LC 215	3
p13	Search in Rotated Sorted Array	Medium	LC 33	3
p14	Maximum Depth of Binary Tree	Easy	LC 104	3
p15	Merge Intervals	Medium	LC 56	3
p16	LRU Cache	Medium	LC 146	4
p17	Number of Islands	Medium	LC 200	4
p18	Coin Change	Medium	LC 322	4
p19	Binary Tree Right Side View	Medium	LC 199	4
p20	Word Search	Medium	LC 79	4

Month 2 — Patterns Mastery (20 problems)

#	Problem	Difficulty	Source	Week
p21	3Sum	Medium	LC 15	5
p22	Trapping Rain Water	Hard	LC 42	5
p23	Subarray Sum Equals K	Medium	LC 560	5
p24	Minimum Size Subarray Sum	Medium	LC 209	5
p25	Maximum Product Subarray	Medium	LC 152	5
p26	Find Minimum in Rotated Sorted Array	Medium	LC 153	6
p27	Daily Temperatures	Medium	LC 739	6
p28	Meeting Rooms II	Medium	LC 253	6
p29	Largest Rectangle in Histogram	Hard	LC 84	6
p30	Jump Game II	Medium	LC 45	6
p31	Course Schedule	Medium	LC 207	7
p32	Pacific Atlantic Water Flow	Medium	LC 417	7
p33	Number of Connected Components	Medium	LC 323	7
p34	Word Ladder	Hard	LC 127	7
p35	Lowest Common Ancestor of BST	Medium	LC 235	7
p36	Combination Sum	Medium	LC 39	8
p37	Unique Paths	Medium	LC 62	8
p38	Implement Trie	Medium	LC 208	8
p39	Find Median from Data Stream	Hard	LC 295	8
p40	Decode Ways	Medium	LC 91	8

Month 3 — Graphs + DP Deep (20 problems)

#	Problem	Difficulty	Source	Week
p41	Network Delay Time	Medium	LC 743	9
p42	Cheapest Flights Within K Stops	Medium	LC 787	9
p43	Shortest Path in Binary Matrix	Medium	LC 1091	9
p44	Word Ladder II	Hard	LC 126	9
p45	Clone Graph	Medium	LC 133	9
p46	Critical Connections in a Network	Hard	LC 1192	10
p47	Alien Dictionary	Hard	LC 269	10
p48	Min Cost to Connect All Points	Medium	LC 1584	10
p49	Is Graph Bipartite?	Medium	LC 785	10
p50	Reconstruct Itinerary	Hard	LC 332	10
p51	Longest Increasing Subsequence	Medium	LC 300	11
p52	Edit Distance	Hard	LC 72	11
p53	Partition Equal Subset Sum	Medium	LC 416	11
p54	Regular Expression Matching	Hard	LC 10	11
p55	Interleaving String	Medium	LC 97	11
p56	Burst Balloons	Hard	LC 312	12
p57	Palindrome Partitioning II	Hard	LC 132	12
p58	Strange Printer	Hard	LC 664	12
p59	Dungeon Game	Hard	LC 174	12
p60	Number of Ways to Wear Different Hats	Hard	LC 1434	12

Month 4 — Greedy, Advanced DS, Practical Engineering (20 problems)

#	Problem	Difficulty	Source	Week
p61	Queue Reconstruction by Height	Medium	LC 406	13
p62	Gas Station	Medium	LC 134	13
p63	Task Scheduler	Medium	LC 621	13
p64	Minimum Number of Arrows	Medium	LC 452	13
p65	Split Array Largest Sum	Hard	LC 410	13
p66	Range Sum Query Mutable	Medium	LC 307	14
p67	Count of Smaller Numbers After Self	Hard	LC 315	14
p68	Implement strStr() (KMP)	Easy	LC 28	14
p69	Repeated DNA Sequences	Medium	LC 187	14
p70	Sliding Window Maximum	Hard	LC 239	14
p71	Design LRU Cache (distributed follow-ups)	Medium	LC 146 + extensions	15
p72	Design Hit Counter	Medium	LC 362	15
p73	Design Autocomplete System	Hard	LC 642	15
p74	LFU Cache	Hard	LC 460	15
p75	ORIGINAL — Consistent Hashing Ring	Hard	Amazon/AWS	15
p76	Design In-Memory File System	Hard	LC 588	16
p77	ORIGINAL — Distributed Rate Limiter	Hard	Meta Infra	16
p78	Text Justification	Hard	LC 68	16
p79	ORIGINAL — Service Mesh Min Deployment	Hard	Google SRE	16
p80	Basic Calculator II	Medium	LC 227	16

Month 5 — Hards + Concurrency + Above-FAANG (10 problems)

#	Problem	Difficulty	Source	Week
p81	Median of Two Sorted Arrays	Hard	LC 4	17
p82	Russian Doll Envelopes	Hard	LC 354	17
p83	First Missing Positive	Hard	LC 41	17
p84	Minimum Window Substring	Hard	LC 76	17
p85	ORIGINAL — Concurrent Web Crawler with Backpressure	Hard	Amazon	18
p86	Design Bounded Blocking Queue	Medium	LC 1188	18
p87	ORIGINAL — S3 Cost-Optimal Eviction	Hard	AWS	18
p88	N-Queens	Hard	LC 51	19
p89	Trapping Rain Water II	Hard	LC 407	19
p90	ORIGINAL — Distributed Job DAG Scheduler	Hard	Google	19

Month 6 — Polish + Mock Marathon

Capstone	Source	Week
p91 — Skyline Problem	LC 218	20
MOCK_MARATHON.md	—	21
RE_SOLVE_GUIDE.md	—	23
FAILURE_PATTERNS.md	—	21–24
FINAL_READINESS.md	—	24

Cross-References to the Rest of the Workspace

FRAMEWORK.md — universal 16-step problem-solving framework
CODE_QUALITY.md — quality bar every solution.py must meet
COMMUNICATION.md — what to say in interviews
FAILURE_ANALYSIS.md — 16-category failure taxonomy
SPACED_REPETITION.md — 6-tier review schedule
READINESS_CHECKLIST.md — final binary checklist
phase-02-patterns/ — concept docs for patterns (3Sum, sliding window, etc.)
phase-04-graphs/ — graph algorithm concept docs
phase-05-dp/ — DP concept docs and labs

Honest Promise

If you complete every problem in this track honestly — Attempt Gate respected, hints used sparingly, follow-ups answered before reading the answers, stress tests passing, every “When to Move On” checklist green — you will be in the top 5% of candidates entering senior, staff, and principal interview loops at Amazon, Google, Meta, Microsoft, Apple, and infrastructure-specialty companies.

If you skip the Attempt Gates, peek at hints early, or skim the Level Delta sections without honestly self-assessing — this is just a list of problems you’ve seen. Seen ≠ owned. Owned is what gets offers.

LeetCode Interview — Extreme Coding