leetcode/README.md

<p align="center">
  <img alt="header" src="https://shieldcn.dev/header/gradient.svg?title=prdlk%2Fleetcode&amp;subtitle=Personal+Leetcode+solutions%2C+concepts%2C+and+patterns&amp;logo=leetcode&amp;mode=dark" />
</p>

<p align="center">
  <a href="https://x.com/basedprad"><img alt="follow" src="https://shieldcn.dev/x/follow/basedprad.svg" /></a>
  <a href="https://lc.prad.nu"><img alt="badge" src="https://shieldcn.dev/badge/Read-Documentation.svg?logo=lu%3ABook" /></a>
</p>


## Phase I — Linear Structures (Week 1)
 
### 1. Hash-Based Lookup — builds on nothing
| Set | LC # | Problem |
|---|---|---|
| A | 217 | Contains Duplicate |
| A | 242 | Valid Anagram |
| A | 1 | Two Sum |
| A | 49 | Group Anagrams |
| A | 347 | Top K Frequent Elements |
| B | 383 | Ransom Note |
| B | 205 | Isomorphic Strings |
| B | 128 | Longest Consecutive Sequence |
| B | 290 | Word Pattern |
 
### 2. Prefix Sum — builds on 1
| Set | LC # | Problem |
|---|---|---|
| A | 303 | Range Sum Query |
| A | 238 | Product of Array Except Self |
| A | 560 | Subarray Sum Equals K |
| B | 525 | Contiguous Array |
| B | 974 | Subarray Sums Divisible by K |
 
### 3. Two Pointers — builds on ordered iteration
| Set | LC # | Problem |
|---|---|---|
| A | 125 | Valid Palindrome |
| A | 167 | Two Sum II |
| A | 15 | 3Sum |
| A | 11 | Container With Most Water |
| A | 42 | Trapping Rain Water |
| B | 392 | Is Subsequence |
| B | 977 | Squares of a Sorted Array |
| B | 189 | Rotate Array |
| B | 80 | Remove Duplicates II |
 
### 4. Sliding Window — builds on 3
| Set | LC # | Problem |
|---|---|---|
| A | 121 | Best Time to Buy and Sell Stock |
| A | 3 | Longest Substring Without Repeating Characters |
| A | 424 | Longest Repeating Character Replacement |
| A | 567 | Permutation in String |
| A | 76 | Minimum Window Substring |
| B | 209 | Minimum Size Subarray Sum |
| B | 1004 | Max Consecutive Ones III |
| B | 643 | Maximum Average Subarray |
 
### 5. Binary Search — builds on ordered/monotonic invariants
| Set | LC # | Problem |
|---|---|---|
| A | 704 | Binary Search |
| A | 74 | Search a 2D Matrix |
| A | 875 | Koko Eating Bananas |
| A | 33 | Search in Rotated Sorted Array |
| A | 153 | Find Minimum in Rotated Sorted Array |
| B | 35 | Search Insert Position |
| B | 162 | Find Peak Element |
| B | 34 | First & Last Position |
| B ⭐ | 4 | Median of Two Sorted Arrays |
 
### 6. Stack — independent primitive
| Set | LC # | Problem |
|---|---|---|
| A | 20 | Valid Parentheses |
| A | 155 | Min Stack |
| A | 150 | Evaluate Reverse Polish Notation |
| B | 71 | Simplify Path |
| B ⭐ | 224 | Basic Calculator |
 
### 7. Monotonic Stack — builds on 6
| Set | LC # | Problem |
|---|---|---|
| A | 739 | Daily Temperatures |
| A | 853 | Car Fleet |
| B | 496 | Next Greater Element I |
| B | 901 | Online Stock Span |
| B ⭐ | 84 | Largest Rectangle in Histogram |
 
---
 
## Phase II — Nodal & Grid Structures (Week 2)
 
### 8. Linked List — independent primitive
| Set | LC # | Problem |
|---|---|---|
| A | 206 | Reverse Linked List |
| A | 21 | Merge Two Sorted Lists |
| A | 141 | Linked List Cycle |
| A | 19 | Remove Nth Node From End |
| A | 143 | Reorder List |
| B | 876 | Middle of the Linked List |
| B | 92 | Reverse Linked List II |
| B | 82 | Remove Duplicates from Sorted List II |
| B | 61 | Rotate List |
| B | 86 | Partition List |
 
### 9. Hybrid Structures — builds on 1 + 8
| Set | LC # | Problem |
|---|---|---|
| A | 146 | LRU Cache |
| A | 2 | Add Two Numbers |
| B | 138 | Copy List with Random Pointer |
| B | 380 | Insert Delete GetRandom O(1) |
 
### 10. Matrix Index Math — builds on 3
| Set | LC # | Problem |
|---|---|---|
| A | 48 | Rotate Image |
| A | 54 | Spiral Matrix |
| A | 73 | Set Matrix Zeroes |
| B | 36 | Valid Sudoku |
| B | 289 | Game of Life |
 
---
 
## Phase III — Hierarchical Structures (Week 3)
 
### 11. Tree DFS — builds on recursion
| Set | LC # | Problem |
|---|---|---|
| A | 226 | Invert Binary Tree |
| A | 104 | Maximum Depth of Binary Tree |
| A | 100 | Same Tree |
| A | 572 | Subtree of Another Tree |
| A | 236 | Lowest Common Ancestor |
| A ⭐ | 124 | Binary Tree Maximum Path Sum |
| B | 101 | Symmetric Tree |
| B | 112 | Path Sum |
| B | 129 | Sum Root to Leaf Numbers |
| B | 543 | Diameter of Binary Tree |
 
### 12. Tree BFS — builds on 6's sibling (queue) + 11
| Set | LC # | Problem |
|---|---|---|
| A | 102 | Level Order Traversal |
| A | 199 | Right Side View |
| A | 1448 | Count Good Nodes |
| B | 103 | Zigzag Level Order |
| B | 637 | Average of Levels |
 
### 13. Binary Search Tree — builds on 5 + 11
| Set | LC # | Problem |
|---|---|---|
| A | 98 | Validate BST |
| A | 230 | Kth Smallest in BST |
| A | 235 | LCA of BST |
| A | 105 | Build from Preorder + Inorder |
| B | 530 | Minimum Absolute Difference |
| B | 173 | BST Iterator |
| B | 108 | Sorted Array to BST |
 
### 14. Heap / Priority Queue — builds on trees (implicit tree over array)
| Set | LC # | Problem |
|---|---|---|
| A | 215 | Kth Largest in Array |
| A | 703 | Kth Largest in Stream |
| A | 973 | K Closest Points |
| A | 621 | Task Scheduler |
| A | 23 | Merge K Sorted Lists |
| B | 1046 | Last Stone Weight |
| B | 373 | K Pairs with Smallest Sums |
| B ⭐ | 295 | Find Median from Data Stream |
| B ⭐ | 502 | IPO |
 
### 15. Trie — builds on 11
| Set | LC # | Problem |
|---|---|---|
| A | 208 | Implement Trie |
| A | 211 | Add & Search Words |
| B | 212 | Word Search II *(needs topic 20 — schedule after)* |
 
---
 
## Phase IV — Relational Structures (Week 4)
 
### 16. Graph DFS — builds on 11 + visited set
| Set | LC # | Problem |
|---|---|---|
| A | 200 | Number of Islands |
| A | 695 | Max Area of Island |
| A | 133 | Clone Graph |
| A | 417 | Pacific Atlantic Water Flow |
| A | 130 | Surrounded Regions |
| B | 399 | Evaluate Division |
| B | 261 | Graph Valid Tree |
 
### 17. Graph BFS — builds on 12 + visited set
| Set | LC # | Problem |
|---|---|---|
| A | 994 | Rotting Oranges |
| A | 127 | Word Ladder |
| B | 909 | Snakes and Ladders |
| B | 433 | Minimum Genetic Mutation |
| B | 286 | Walls and Gates |
 
### 18. Topological Sort — builds on 16
| Set | LC # | Problem |
|---|---|---|
| A | 207 | Course Schedule |
| A | 210 | Course Schedule II |
| B ⭐ | 269 | Alien Dictionary |
| B | 1462 | Course Schedule IV |
 
### 19. Union-Find — builds on 16's problem class
| Set | LC # | Problem |
|---|---|---|
| A | 684 | Redundant Connection |
| A | 323 | Number of Connected Components |
| B | 128 | Longest Consecutive (union-find variant) |
| B | 721 | Accounts Merge |
 
---
 
## Phase V — Decision-Space Exploration (Weeks 4–5)
 
### 20. Backtracking — builds on 16
| Set | LC # | Problem |
|---|---|---|
| A | 78 | Subsets |
| A | 39 | Combination Sum |
| A | 46 | Permutations |
| A | 79 | Word Search |
| B | 17 | Letter Combinations |
| B | 77 | Combinations |
| B | 22 | Generate Parentheses |
| B ⭐ | 52 | N-Queens II |
 
*(Now unlock 212 Word Search II from topic 15.)*
 
### 21. 1-D Dynamic Programming — builds on 20
| Set | LC # | Problem |
|---|---|---|
| A | 70 | Climbing Stairs |
| A | 746 | Min Cost Climbing Stairs |
| A | 198 | House Robber |
| A | 213 | House Robber II |
| A | 322 | Coin Change |
| A | 300 | Longest Increasing Subsequence |
| B | 139 | Word Break |
| B | 91 | Decode Ways |
| B | 647 | Palindromic Substrings |
 
### 22. Multi-D / Grid DP — builds on 21
| Set | LC # | Problem |
|---|---|---|
| A | 62 | Unique Paths |
| A | 64 | Minimum Path Sum |
| A | 1143 | Longest Common Subsequence |
| B | 120 | Triangle |
| B | 63 | Unique Paths II |
| B ⭐ | 72 | Edit Distance |
| B | 221 | Maximal Square |
| B ⭐ | 97 | Interleaving String |
 
### 23. Greedy — builds on 21
| Set | LC # | Problem |
|---|---|---|
| A | 53 | Maximum Subarray (Kadane's) |
| A | 55 | Jump Game |
| A | 45 | Jump Game II |
| A | 134 | Gas Station |
| B | 122 | Best Time to Buy and Sell Stock II |
| B | 918 | Maximum Sum Circular Subarray |
| B | 763 | Partition Labels |
 
### 24. Intervals — builds on 23
| Set | LC # | Problem |
|---|---|---|
| A | 57 | Insert Interval |
| A | 56 | Merge Intervals |
| A | 435 | Non-Overlapping Intervals |
| A | 253 | Meeting Rooms II |
| B | 228 | Summary Ranges |
| B | 452 | Minimum Arrows to Burst Balloons |
| B | 920 | Meeting Rooms I |