Practice Problems — 21 Flagship Deep Dives (one per core pattern)

The 21 core patterns each get their canonical problem walked through brute force → intermediate → optimized. Every approach has its own interactive Execution Trace so you can see how each algorithm actually runs on the same example input.

Why walk the ladder instead of jumping to the optimized solution?

That's what interviews measure. The optimized solution is 20% of the grade; the walk from brute → optimized is 60%. Being able to articulate why the brute force is too slow and what specific insight unlocks the next approach is the senior signal that gets you leveled up.

How each page is structured

1. PROBLEM        — statement, examples, constraints
2. APPROACH 1     — Brute Force
   ├─ Intuition    │ Java code │ Execution Trace │ Complexity
3. APPROACH 2     — Intermediate improvement
   ├─ Insight from brute
4. APPROACH 3     — Optimized
   ├─ Insight from intermediate
5. COMPLEXITY SUMMARY — one comparison table
6. WHEN TO USE WHICH  — interviewer-signal decision guide
7. RELATED PROBLEMS   — variations that reuse the same ladder

The 21 pages

Part A — Windows & Pointers

• 1. Sliding Window — Longest Substring Without Repeating Characters
• 2. Two Pointers — Container With Most Water
• 3. Fast/Slow Pointers — Linked List Cycle II
• 4. Prefix Sum — Subarray Sum Equals K
• 5. Hashing — Two Sum
• 6. Monotonic Stack — Daily Temperatures

Part B — Search & Order

• 7. Binary Search — Search in Rotated Sorted Array
• 8. Binary Search on Answer — Koko Eating Bananas
• 9. Top-K / Heap — Top K Frequent Elements
• 10. K-way Merge — Merge K Sorted Lists
• 11. Merge Intervals — Merge Intervals
• 12. Sweep Line — Meeting Rooms II

Part C — Graphs & Sets

• 13. Topological Sort — Course Schedule II
• 14. Union-Find — Number of Provinces

Part D — Greedy & Recursion

• 15. Greedy — Jump Game II
• 16. Backtracking — N-Queens
• 17. Divide & Conquer — Count Inversions

Part E — Dynamic Programming

• 18. Dynamic Programming — House Robber

Part F — Specialized

• 19. Trie — Word Search II
• 20. Bit Manipulation — Single Number
• 21. Quickselect — Kth Largest Element

The pattern chapters vs these pages

You want to…	Use…
Learn the pattern first time	Pattern chapters (Part II)
Practice a specific problem	These pages
See multiple approaches to one problem	These pages
See a pattern applied across many problems	Pattern chapters
Revise before interview	Pattern chapters + cheat sheets

Both cross-link. Every problem page links to its pattern chapter; every pattern chapter links to its canonical problem.