73 - The best number

According to Sheldon Cooper the best number is 73, because 73 is the 21st prime number. Its mirror, 37, is the 12th prime number. 21 is the product of multiplying 7 by 3 and in binary, 73 is a palindrome: 1001001

Sheldon Cooper (character in the famous series "The Big-Bang Theory")

What is this?

This is a repository that will take you by the hand in the 73 most useful LeetCode problems, well they are actually 76, 😬 the algorithms are written in C# and F#, I wrote each program in the most optimal way and included the main function or method of the program to test on your machine, a README file with solution notes and for problems 252, 253, 269, 271 and 323 being Premium subscription problems, I included a README with the description of each problem.

No hurry, but no rest

Repository structure

The repository is divided into 4 folders:

|- Repository
    |- Problems
        |- Arrays
        |- Matrix
            |- Rotate-Image
            |- Set-Matrix-Zeroes
            |- Spiral-Matrix
            |- Word-Search
                |- Word-Search.cs
                |- Word-Search.csproj
                |- Word-Search.fsx
                |- README.md
        |- Strings
        |- Intervals
        |- Binary Search
        |- Dynamic Programming
        |- Linked Lists
        |- Trees
        |- Heaps
        |- Graphs
    |- Sources
        |- Sheldon.gif

• Problems: Contains the folders for each topic, inside it has a folder per problem and inside each folder is the source code and the README that explains the problem, for example the Two-Sum folder contains the C# source code in Two-Sum.cs and the .NET project settings in Two-Sum.csproj (only the last problem of each topic also includes an F# solution, e.g. Word-Search.fsx) and finally the README.md file. Every problem's README.md also ends with a Suggested practice section linking 3 related LeetCode problems, so you can practice the same pattern

• Sources: Contains the images and gifs used in the repository, such as Sheldon Cooper's gif

Index

Arrays

#	Title	Solution	Difficulty
0001	Two Sum	C#	Easy
0011	Container With Most Water	C#	Medium
0015	3Sum	C#	Medium
0033	Search in Rotated Sorted Array	C#	Medium
0053	Maximum Subarray	C#	Medium
0121	Best Time to Buy and Sell Stock	C#	Medium
0152	Maximum Product Subarray	C#	Medium
0153	Find Minimum in Rotated Sorted Array	C#	Medium
0217	Contains Duplicate	C#	Easy
0238	Product of Array Except Self	C# - F#	Easy

Matrix

#	Title	Solution	Difficulty
0048	Rotate Image	C#	Medium
0054	Spiral Matrix	C#	Medium
0073	Set Matrix Zeroes	C#	Medium
0079	Word Search	C# - F#	Medium

Strings

#	Title	Solution	Difficulty
0003	Longest Substring Without Repeating Characters	C#	Medium
0005	Longest Palindromic Substring	C#	Medium
0020	Valid Parentheses	C#	Easy
0049	Group Anagrams	C#	Medium
0076	Minimum Window Substring	C#	Hard
0125	Valid Palindrome	C#	Easy
0242	Valid Anagram	C#	Easy
0271	Encode and Decode Strings	C#	Medium
0424	Longest Repeating Character Replacement	C#	Medium
0647	Palindromic Substrings	C# - F#	Medium

Intervals

#	Title	Solution	Difficulty
0056	Merge Intervals	C#	Medium
0057	Insert Interval	C#	Medium
0252	Meeting Rooms	C#	Easy
0253	Meeting Rooms II	C#	Medium
0435	Non-overlapping Intervals	C# - F#	Medium

Binary

#	Title	Solution	Difficulty
0190	Reverse Bits	C#	Easy
0191	Number of 1 Bits	C#	Easy
0268	Missing Number	C#	Easy
0338	Counting Bits	C#	Easy
0371	Sum of Two Integers	C# - F#	Medium

Dynamic Programming

#	Title	Solution	Difficulty
0055	Jump Game	C#	Medium
0062	Unique Paths	C#	Medium
0070	Climbing Stairs	C#	Easy
0091	Decode Ways	C#	Medium
0139	Word Break	C#	Medium
0198	House Robber	C#	Medium
0213	House Robber II	C#	Medium
0300	Longest Increasing Subsequence	C#	Medium
0322	Coin Change	C#	Medium
0377	Combination Sum IV	C#	Medium
1143	Longest Common Subsequence	C# - F#	Medium

Linked Lists

#	Title	Solution	Difficulty
0019	Remove Nth Node From End of List	C#	Medium
0021	Merge Two Sorted Lists	C#	Easy
0023	Merge k Sorted Lists	C#	Hard
0141	Linked List Cycle	C#	Easy
0143	Reorder List	C#	Medium
0206	Reverse Linked List	C# - F#	Easy

Trees

#	Title	Solution	Difficulty
0098	Validate Binary Search Tree	C#	Medium
0100	Same Tree	C#	Easy
0102	Binary Tree Level Order Traversal	C#	Medium
0104	Maximum Depth of Binary Tree	C#	Easy
0105	Construct Binary Tree from Preorder and Inorder Traversal	C#	Medium
0124	Binary Tree Maximum Path Sum	C#	Hard
0208	Implement Trie (Prefix Tree)	C#	Medium
0211	Design Add and Search Words Data Structure	C#	Medium
0212	Word Search II	C#	Hard
0226	Invert Binary Tree	C#	Hard
0230	Kth Smallest Element in a BST	C#	Medium
0235	Lowest Common Ancestor of a Binary Search Tree	C#	Medium
0297	Serialize and Deserialize Binary Tree	C#	Hard
0572	Subtree of Another Tree	C# - F#	Easy

Heaps

#	Title	Solution	Difficulty
0295	Find Mediumn from Data Stream	C#	Hard
0347	Top K Frequent Elements	C#	Medium
0355	Design Twitter	C# - F#	Medium

Graphs

#	Title	Solution	Difficulty
0128	Longest Consecutive Sequence	C#	Medium
0133	Clone Graph	C#	Medium
0200	Number of Islands	C#	Medium
0207	Course Schedule	C#	Medium
0261	Graph Valid Tree	C#	Medium
0269	Alien Dictionary	C#	Hard
0323	Number of Connected Components in an Undirected Graph	C#	Medium
0417	Pacific Atlantic Water Flow	C# - F#	Medium

Configuration

The focus of this repository is towards C#, so you can use it from Windows, Linux, Mac, Docker or even GitHub Codespaces, but it is intended to be used on Linux, specifically an Ubuntu based distribution or Red Hat based distribution, no matter if it is a complete distro or a WSL distro. All solutions are written in C#, and only the last problem of each topic also includes a solution in F#.

Prerequisites

• dotnet-sdk-10.0 — required to build and run both the C# and F# solutions.

Ubuntu 22.04 or later:

sudo apt update && \
    sudo apt install -y dotnet-sdk-10.0

Fedora 39 or later:

sudo dnf install -y dotnet-sdk-10.0

Note: To install on other Linux versions or distributions, Windows or macOS, please check this page.

Docker: run everything inside the official SDK image, mounting the repository as a volume:

docker run --rm -it -v "${PWD}:/repo" -w /repo mcr.microsoft.com/dotnet/sdk:10.0 bash

GitHub Codespaces: open the repository in a Codespace, the container image ships with dotnet-sdk-10.0 already installed, so you can build and test right away with no extra setup.

Getting started

1. Clone the repository:

   git clone https://github.com/jonas1ara/73.git
   cd 73

2. Restore dependencies:

   dotnet restore 73.sln

3. Verify your SDK version:

   dotnet --version

Visual Studio

If you are on Windows and use this repository with Visual Studio, you just have to change the solution to compile: open 73.sln, pick the desired project in the Solution Explorer and set it as the startup project (or select it from the run configuration dropdown) to build and run it.

Tests

Every problem has a matching <Problem>.Tests xUnit project, all registered in 73.sln, and they also run automatically on every push/PR to main via the Tests GitHub Actions workflow.

Run the full suite (all problems)

dotnet test 73.sln

Run only one problem's tests

dotnet test Problems/<Topic>/<Problem>.Tests/<Problem>.Tests.csproj

For example:

dotnet test Problems/Graphs/Course-Schedule.Tests/Course-Schedule.Tests.csproj

Run a single test by name

dotnet test 73.sln --filter "FullyQualifiedName~CanFinish_DetectsWhetherAllCoursesCanBeCompleted"

Run with code coverage

dotnet test 73.sln --collect:"XPlat Code Coverage"

Run from Visual Studio

1. Open 73.sln.
2. Go to Test → Test Explorer (or press Ctrl+E, T).
3. Click Run All Tests to run the whole solution, or right-click a specific test/project in Test Explorer and choose Run to run just that one.

Build with

• C# and F#
• Windows
• Ubuntu on WSL
• VS Code and Visual Studio

C# Conferences

• High Speed History of C#
• Mono Edge - Miguel de Icaza
• .NET at Stack Overflow | .NET Conf 2022
• Miguel de Icaza's Vision Keynote - UnoConf 2019
• Full stack web UI with Blazor in .NET 8 | .NET Conf 2023
• Patterns for high-performance C# - Federico Andres Lois
• Let's design a new C# language feature! | .NET Conf 2022
• Adam SITNIK: Solving .NET performance issues | UCP2019
• What’s Next in C#? - Mads Torgersen - Copenhagen DevFest 2023
• S101 - Keynote with Scott Hunter, Mads Torgersen, and Miguel De Icaza
• The functional journey of C# - Mads Torgersen - NDC Copenhagen 2022
• Stop using Entity Framework as a DTO provider! - Chris Klug - NDC Oslo 2023
• You are doing logging in .NET wrong. Let’s fix it. - Nick Chapsas - NDC Oslo 2023
• .NET Systems Programming Learned the Hard Way - Aaron Stannard - NDC Oslo 2023
• Keynote: The Microsoft Open Source Cinematic Universe - Phase 2 - Scott Hanselman
• Using C# Source Generators to Build a .NET IoT Device - Alon Fliess - NDC Melbourne 2022
• Patterns for application development with ASP.NET Core - Damian Edwards & David Fowler
• Performance tricks I learned from contributing to open source .NET packages - Daniel Marbach
• Real-world examples on optimizing .NET performance - Stefán Jökull Sigurðarson - NDC Oslo 2023

References

F# Docs - Get started, Tutorials, reference. https://fsharp.org/docs/

C# Docs - Get started, Tutorials, reference. https://learn.microsoft.com/en-us/dotnet/csharp/

Cormen, T. H., Leiserson, C. E., Rivest, R. L. & Stein, C. (2009). Introduction to Algorithms (3rd ed.). MIT Press.

License

This project is licensed under the MIT License.

Acknowledgments

Written with ❤️ since December 2022 by Jonas Lara for anyone who is interested — started in a time without AI, improved in the age of AI.