Case File Library

Work through LeetCode-style data structure and algorithm investigations that teach you how to read problem statements, follow constraints, eliminate suspects, qualify candidates, and prove the correct solution before writing code.

Correct Code Framework Case File System hero image
Learning Path

Case Files are the practice engine.

Published today: Case Files #001-#014. Case Files #001-#006 are public, and #007-#014 are included with Starter Plan or higher. Later entries are the future curriculum roadmap.

  1. Understand the Framework Learn Answer Space, Problem Space, and Decision Space.
  2. Solve Case File #001 Apply the method to the first investigation.
  3. Continue Through Public Case Files Work the first 6 public investigations before paid access. You are here
  4. Unlock Book Zero / Starter Path Continue into the paid Case File path when you are ready.
  5. Study Companion Books Deepen the principles behind the practice.
  6. Expand Into SQL Map CCF to declarative dataset reasoning.
  7. Advance Into Enterprise Use CCF for systems, cloud, DevOps, and architecture.
New to CCF?

Start with Case File #001 before browsing the full library.

The library is large on purpose, but the first move is small: open Target Element, learn the investigation workflow, then use the Reasoning Guide to name the structure before continuing.

Start Case File #001 Get the Reasoning Guide
Practice With Purpose

Why These Case Files Matter

Case Files are not riddles to memorize. Each problem is a controlled environment for practicing how to identify state, derive an invariant, determine what must persist, reason about lawful transitions, evaluate constraints, and prove a solution before code.

As the problems become more difficult, the reasoning becomes richer, but the underlying discipline remains consistent. The goal is not to become good at solving isolated puzzles. It is to develop reasoning that transfers to larger engineering problems.

Practice the reasoning at algorithm scale. Apply it at system scale.

Student develops the reasoning at algorithm scale. Pro applies that discipline at system scale.

Public Preview Published Case Files #001-#006
Starter Plan Published #007-#150; roadmap through #045

#007 House Robber
#008 Clone Graph
#009 Coin Change

#010 Level Order Traversal
#011 Valid Parentheses
#012 Daily Temperatures

#013 Merge Intervals
#014 Search in Rotated Sorted Array
#015 Kth Largest Element

#016 Top K Frequent Elements
#017 Number of Islands
#018 Course Schedule

#019 Lowest Common Ancestor
#020 Validate Binary Search Tree
#021 Subsets

#022 Combination Sum
#023 Longest Increasing Subsequence
#024 Edit Distance

#025 Word Break

#026 Reverse Linked List
#027 Linked List Cycle

#028 Merge K Sorted Lists
#029 LRU Cache
#030 Trapping Rain Water

#031 Largest Rectangle in Histogram
#032 Find Median from Data Stream
#033 Implement Trie

#034 Word Search II
#035 Accounts Merge
#036 Network Delay Time

#037 Cheapest Flights Within K Stops
#038 Min Cost to Connect All Points
#039 Serialize and Deserialize Binary Tree

#040 Binary Tree Maximum Path Sum
#041 Kth Smallest Element in a BST
#042 Construct Tree from Traversals

#043 N-Queens
#044 Palindrome Partitioning
#045 Decode Ways
Student Plan — DSA Curriculum Future curriculum roadmap #046-#075

#046 Partition Equal Subset Sum
#047 Longest Common Subsequence
#048 Burst Balloons

#049 Regular Expression Matching
#050 Alien Dictionary

#051 Contains Duplicate
#052 Product of Array Except Self
#053 Maximum Subarray

#054 Maximum Product Subarray
#055 Find Minimum in Rotated Sorted Array
#056 Container With Most Water

#057 Sum of Two Integers
#058 Number of 1 Bits
#059 Counting Bits

#060 Missing Number
#061 Reverse Bits
#062 Climbing Stairs

#063 Unique Paths
#064 Jump Game
#065 Longest Repeating Character Replacement

#066 Group Anagrams
#067 Longest Consecutive Sequence
#068 Insert Interval

#069 Non-overlapping Intervals
#070 Meeting Rooms II
#071 Remove Nth Node From End of List

#072 Same Tree
#073 Subtree of Another Tree
#074 Invert Binary Tree

#075 Pacific Atlantic Water Flow
Pro Plan — Systems Reasoning Curriculum Future curriculum roadmap #076-#150

#076 Minimum Size Subarray Sum
#077 Longest Substring Without Repeating Characters
#078 Permutation in String
#079 Find All Anagrams in a String
#080 Longest Substring with At Most K Distinct Characters

#081 4 Sum
#082 3 Sum Closest
#083 Container With Most Water II
#084 Valid Palindrome II
#085 Trapping Rain Water II

#086 Find K Closest Elements
#087 Kth Largest Element in an Array
#088 Top K Frequent Words
#089 Merge K Sorted Arrays
#090 Sliding Window Maximum

#091 Word Ladder
#092 Word Ladder II
#093 Reconstruct Itinerary
#094 Critical Connections in a Network
#095 Redundant Connection II

#096 Number of Connected Components
#097 Graph Valid Tree
#098 Satisfiability of Equality Equations
#099 Number of Provinces
#100 Similar String Groups

#101 Best Time to Buy and Sell Stock III
#102 Best Time to Buy and Sell Stock IV
#103 House Robber II
#104 Decode Ways II
#105 Wildcard Matching

#106 Regular Expression Matching II
#107 Interleaving String
#108 Distinct Subsequences
#109 Dungeon Game
#110 Minimum Path Sum II

#111 Binary Tree Serialization Compression
#112 Vertical Order Traversal
#113 Binary Tree Cameras
#114 Maximum Width of Binary Tree
#115 Recover Binary Search Tree

#116 Median of Two Sorted Arrays
#117 Search a 2D Matrix II
#118 Find Peak Element
#119 Capacity To Ship Packages Within D Days
#120 Split Array Largest Sum

#121 Sudoku Solver
#122 Restore IP Addresses
#123 Expression Add Operators
#124 Letter Tile Possibilities
#125 Matchsticks to Square

#126 Implement strStr (KMP)
#127 Longest Palindromic Substring
#128 Longest Palindromic Subsequence
#129 Shortest Palindrome
#130 Text Justification

#131 Spiral Matrix II
#132 Rotate Image
#133 Set Matrix Zeroes
#134 Game of Life
#135 Maximal Rectangle

#136 LFU Cache
#137 All O(1) Data Structure
#138 Randomized Set
#139 Insert Delete GetRandom O(1)
#140 Design Twitter

#141 Design LRU Cache II
#142 Design Browser History
#143 Design Underground System
#144 Time Based Key Value Store
#145 Snapshot Array

#146 Median Finder Architecture
#147 Data Stream as Disjoint Intervals
#148 Sliding Window Median
#149 Minimum Cost Maximum Flow
#150 The Skyline Problem