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Hand-written tutorials, ordered as a recommended learning path.

1. 181 Anagram Patterns Master anagram problems — valid anagram checks, grouping anagrams by sorted and frequency keys, finding all anagrams in a string with sliding windows, and the minimum window substring problem. 11 min read →
2. 182 Subsequences Master string subsequence problems — check subsequences with two pointers, count distinct subsequences with DP, find the longest common subsequence, and build the shortest common supersequence. 13 min read →
3. 183 Parentheses Master parentheses problems — validate with stacks, generate all combinations with backtracking, find the longest valid substring with DP and stack, remove invalid parentheses with BFS, and more. 13 min read →
4. 184 Encode/Decode Master string encoding and decoding — delimiter-based encode/decode, run-length encoding, decoding nested bracket strings with stacks, string compression, and serialization patterns. 12 min read →
5. 185 String Sliding Window Sliding window on strings with reusable templates — longest substring without repeating, minimum window substring, K distinct characters, and permutation check. 11 min read →
6. 186 String Tricks Essential string manipulation tricks — reverse words, reverse vowels, string rotation check, repeated substring pattern, multiply strings, and add binary. 11 min read →
7. 187 Trie Applications Advanced Trie applications — autocomplete system, search suggestions, word break with Trie, longest word in dictionary, replace words, and magic dictionary. 14 min read →
8. 188 String Hashing Master string hashing for pattern matching — polynomial hashing, rolling hash for Rabin-Karp, double hashing, repeated DNA sequences, and longest duplicate substring. 15 min read →
9. 189 String Interview Guide A complete guide to string interview patterns — top 20 patterns, two-pointer on strings, frequency map technique, sliding window template, when to use Trie vs HashMap, common mistakes, and a complexity cheatsheet. 15 min read →
10. 190 DP on Trees Master dynamic programming on trees — max independent set, tree diameter via DP, House Robber III, rerooting technique, and post-order traversal patterns with Python implementations. 15 min read →
11. 191 DP on Strings Master string dynamic programming — Longest Common Subsequence, Edit Distance, Longest Palindromic Subsequence, Wildcard and Regex Matching with Python implementations and space optimization. 13 min read →
12. 192 Bitmask DP Master bitmask dynamic programming — represent subsets as integers, solve the Travelling Salesman Problem, assignment problem, and subset enumeration with Python implementations. 13 min read →
13. 193 DP State Machine Model dynamic programming as state machines — solve all Buy and Sell Stock variants (I-IV, cooldown, fee), understand state transitions, and build a general framework with Python. 11 min read →
14. 194 Counting & Combinatorics Master counting techniques for DSA: permutations, combinations, Pascal's triangle, Catalan numbers, inclusion-exclusion, pigeonhole principle, and modular arithmetic. 16 min read →
15. 195 Knapsack DP Variants Master every knapsack variant — 0/1 knapsack, unbounded knapsack, fractional knapsack, subset sum, partition equal subset, and target sum with Python solutions and Big-O analysis. 10 min read →
16. 196 DP Grid Traversal Master DP on grids — unique paths, minimum path sum, dungeon game, cherry pickup, and maximum path in grid with step-by-step Python solutions. 9 min read →
17. 197 DP Palindrome Problems Master palindrome DP problems — longest palindromic subsequence, minimum cuts for palindrome partitioning, counting palindromic substrings, and shortest palindrome with KMP. 10 min read →
18. 198 Stock Trading Complete Complete guide to all stock trading problems — I through IV, with cooldown, and with transaction fee. One unified state machine DP framework covers them all. 8 min read →
19. 199 Longest Subsequence Variants Master longest subsequence problems — LIS with patience sorting, longest bitonic, chain of pairs, zigzag subsequence, Russian doll envelopes (2D LIS). 9 min read →
20. 200 Monotonic Stack Guide Master the monotonic stack pattern — next greater/smaller element, largest rectangle in histogram, maximal rectangle, trapping rain water, stock span, daily temperatures. 9 min read →