interactive-coding-challenges

120+ continually updated, interactive, and test-driven coding challenges, with Anki flashcards.

Challenges focus on algorithms and data structures found in coding interviews.

Each challenge has one or more reference solutions that are:

Fully functional
Unit tested
Easy-to-understand

Challenges will soon provide on-demand incremental hints to help you arrive at the optimal solution.

Notebooks also detail:

Constraints
Test cases
Algorithms
Big-O time and space complexities

Also included are unit tested reference implementations of various data structures and algorithms.

Challenge Solutions

Anki Flashcards: Coding and Design

The provided Anki flashcard deck uses spaced repetition to help you retain key concepts.

Coding deck

Great for use while on-the-go.

Design Resource: The System Design Primer

Looking for resources to help you prep for the System Design and Object-Oriented Design interviews?

Check out the sister repo The System Design Primer, which contains additional Anki decks:

Notebook Structure

Each challenge has two notebooks, a challenge notebook with unit tests for you to solve and a solution notebook for reference.

Problem Statement

States the problem to solve.

Constraints

Describes any constraints or assumptions.

Test Cases

Describes the general and edge test cases that will be evaluated in the unit test.

Algorithm

[Challenge Notebook] Empty, refer to the solution notebook algorithm section if you need a hint.
[Solution Notebook] One or more algorithm solution discussions, with Big-O time and space complexities.

Hints

[Challenge Notebook] Provides on-demand incremental hints to help you arrive at the optimal solution. Coming soon!

Code (Challenge: Implement Me!)

[Challenge Notebook] Skeleton code for you to implement.
[Solution Notebook] One or more reference solutions.

Unit Test

[Challenge Notebook] Unit test for your code. Expected to fail until you solve the challenge.
[Solution Notebook] Unit test for the reference solution(s).

Index

Challenges Categories

Format: Challenge Category - Number of Challenges

Arrays and Strings - 10
Linked Lists - 8
Stacks and Queues - 8
Graphs and Trees - 21
Sorting - 10
Recursion and Dynamic Programming - 17
Mathematics and Probability - 6
Bit Manipulation - 8
Online Judges - 16
System Design - 8
Object Oriented Design - 8

Total number of challenges: 120

Reference Implementations: Data Structures

Unit tested, fully functional implementations of the following data structures:

Reference Implementations: Algorithms

Unit tested, fully functional implementations of the following algorithms:

Reference Implementations: TODO

Misc

Contributing
Credits
Contact Info
License

Challenges

Image Credits

Arrays and Strings

Challenge	Static Notebook
Determine if a string contains unique characters	Challenge │ Solution
Determine if a string is a permutation of another	Challenge │ Solution
Determine if a string is a rotation of another	Challenge │ Solution
Compress a string	Challenge │ Solution
Reverse characters in a string	Challenge │ Solution
Given two strings, find the single different char	Challenge │ Solution
Find two indices that sum to a specific value	Challenge │ Solution
Implement a hash table	Challenge │ Solution
Implement fizz buzz	Challenge │ Solution
Find the first non-repeated character in a string	Contribute │ Contribute
Remove specified characters in a string	Contribute │ Contribute
Reverse words in a string	Contribute │ Contribute
Convert a string to an integer	Contribute │ Contribute
Convert an integer to a string	Contribute │ Contribute
Add a challenge	Contribute │ Contribute

Linked Lists

Challenge	Static Notebook
Remove duplicates from a linked list	Challenge │ Solution
Find the kth to last element of a linked list	Challenge │ Solution
Delete a node in the middle of a linked list	Challenge │ Solution
Partition a linked list around a given value	Challenge │ Solution
Add two numbers whose digits are stored in a linked list	Challenge │ Solution
Find the start of a linked list loop	Challenge │ Solution
Determine if a linked list is a palindrome	Challenge │ Solution
Implement a linked list	Challenge │ Solution
Determine if a list is cyclic or acyclic	Contribute │ Contribute
Add a challenge	Contribute │ Contribute

Stacks and Queues

Challenge	Static Notebook
Implement n stacks using a single array	Challenge │ Solution
Implement a stack that keeps track of its minimum element	Challenge │ Solution
Implement a set of stacks class that wraps a list of capacity-bounded stacks	Challenge │ Solution
Implement a queue using two stacks	Challenge │ Solution
Sort a stack using another stack as a buffer	Challenge │ Solution
Implement a stack	Challenge │ Solution
Implement a queue	Challenge │ Solution
Implement a priority queue backed by an array	Challenge │ Solution
Add a challenge	Contribute │ Contribute

Graphs and Trees

Challenge	Static Notebooks
Implement depth-first search (pre-, in-, post-order) on a tree	Challenge │ Solution
Implement breadth-first search on a tree	Challenge │ Solution
Determine the height of a tree	Challenge │ Solution
Create a binary search tree with minimal height from a sorted array	Challenge │ Solution
Create a linked list for each level of a binary tree	Challenge │ Solution
Check if a binary tree is balanced	Challenge │ Solution
Determine if a tree is a valid binary search tree	Challenge │ Solution
Find the in-order successor of a given node in a binary search tree	Challenge │ Solution
Find the second largest node in a binary search tree	Challenge │ Solution
Find the lowest common ancestor	Challenge │ Solution
Invert a binary tree	Challenge │ Solution
Implement a binary search tree	Challenge │ Solution
Implement a min heap	Challenge │ Solution
Implement a trie	Challenge │ Solution
Implement depth-first search on a graph	Challenge │ Solution
Implement breadth-first search on a graph	Challenge │ Solution
Determine if there is a path between two nodes in a graph	Challenge │ Solution
Implement a graph	Challenge │ Solution
Find a build order given a list of projects and dependencies.	Challenge │ Solution
Find the shortest path in a weighted graph.	Challenge │ Solution
Find the shortest path in an unweighted graph.	Challenge │ Solution
Add a challenge	Contribute │ Contribute

Sorting

Challenge	Static Notebooks
Implement selection sort	Challenge │ Solution
Implement insertion sort	Challenge │ Solution
Implement quick sort	Challenge │ Solution
Implement merge sort	Challenge │ Solution
Implement radix sort	Challenge │ Solution
Sort an array of strings so all anagrams are next to each other	Challenge │ Solution
Find an item in a sorted, rotated array	Challenge │ Solution
Search a sorted matrix for an item	Challenge │ Solution
Find an int not in an input of n integers	Challenge │ Solution
Given sorted arrays A, B, merge B into A in sorted order	Challenge │ Solution
Implement a stable selection sort	Contribute │ Contribute
Make an unstable sort stable	Contribute │ Contribute
Implement an efficient, in-place version of quicksort	Contribute │ Contribute
Given two sorted arrays, merge one into the other in sorted order	Contribute │ Contribute
Find an element in a rotated and sorted array of integers	Contribute │ Contribute
Add a challenge	Contribute │ Contribute

Recursion and Dynamic Programming

Challenge	Static Notebooks
Implement fibonacci recursively, dynamically, and iteratively	Challenge │ Solution
Maximize items placed in a knapsack	Challenge │ Solution
Maximize unbounded items placed in a knapsack	Challenge │ Solution
Find the longest common subsequence	Challenge │ Solution
Find the longest increasing subsequence	Challenge │ Solution
Minimize the cost of matrix multiplication	Challenge │ Solution
Maximize stock prices given k transactions	Challenge │ Solution
Find the minimum number of ways to represent n cents given an array of coins	Challenge │ Solution
Find the unique number of ways to represent n cents given an array of coins	Challenge │ Solution
Print all valid combinations of n-pairs of parentheses	Challenge │ Solution
Navigate a maze	Challenge │ Solution
Print all subsets of a set	Challenge │ Solution
Print all permutations of a string	Challenge │ Solution
Find the magic index in an array	Challenge │ Solution
Find the number of ways to run up n steps	Challenge │ Solution
Implement the Towers of Hanoi with 3 towers and N disks	Challenge │ Solution
Implement factorial recursively, dynamically, and iteratively	Contribute │ Contribute
Perform a binary search on a sorted array of integers	Contribute │ Contribute
Print all combinations of a string	Contribute │ Contribute
Implement a paint fill function	Contribute │ Contribute
Find all permutations to represent n cents, given 1, 5, 10, 25 cent coins	Contribute │ Contribute
Add a challenge	Contribute │ Contribute

Mathematics and Probability

Challenge	Static Notebooks
Generate a list of primes	Challenge │ Solution
Find the digital root	Challenge │ Solution
Create a class supporting insert, max, min, mean, mode in O(1)	Challenge │ Solution
Determine if a number is a power of two	Challenge │ Solution
Add two numbers without the + or - sign	Challenge │ Solution
Subtract two numbers without the + or - sign	Challenge │ Solution
Check if a number is prime	Contribute │ Contribute
Determine if two lines on a Cartesian plane intersect	Contribute │ Contribute
Using only add, implement multiply, subtract, and divide for ints	Contribute │ Contribute
Find the kth number such that the only prime factors are 3, 5, and 7	Contribute │ Contribute
Add a challenge	Contribute │ Contribute

Bit Manipulation

Challenge	Static Notebooks
Implement common bit manipulation operations	Challenge │ Solution
Determine number of bits to flip to convert a into b	Challenge │ Solution
Draw a line on a screen	Challenge │ Solution
Flip a bit to maximize the longest sequence of 1s	Challenge │ Solution
Get the next largest and next smallest numbers	Challenge │ Solution
Merge two binary numbers	Challenge │ Solution
Swap odd and even bits in an integer	Challenge │ Solution
Print the binary representation of a number between 0 and 1	Challenge │ Solution
Determine the number of 1s in the binary representation of a given integer	Contribute │ Contribute
Add a challenge	Contribute │ Contribute

Online Judges

Challenge	Static Notebooks
Find the longest substring with at most k distinct chars	Challenge │ Solution
Find the highest product of three numbers	Challenge │ Solution
Maximize stocks profit from 1 buy and 1 sell	Challenge │ Solution
Move all zeroes in a list to the end	Challenge │ Solution
Find the products of every other int	Challenge │ Solution
Given a list of entries and exits, find the busiest period	Challenge │ Solution
Determine an island’s perimeter	Challenge │ Solution
Format license keys	Challenge │ Solution
Find the longest absolute file path	Challenge │ Solution
Merge tuple ranges	Challenge │ Solution
Assign cookies	Challenge │ Solution
Determine if you can win in Nim	Challenge │ Solution
Check if a magazine could have been used to create a ransom note	Challenge │ Solution
Find the number of times a sentence can fit on a screen	Challenge │ Solution
Utopian tree	Challenge │ Solution
Maximizing xor	Challenge │ Solution
Add a challenge	Contribute │ Contribute

Repo Structure

interactive-coding-challenges        # Repo
├─ arrays_strings                    # Category of challenges
│  ├─ rotation                       # Challenge folder
│  │  ├─ rotation_challenge.ipynb    # Challenge notebook
│  │  ├─ rotation_solution.ipynb     # Solution notebook
│  │  ├─ test_rotation.py            # Unit test*
│  ├─ compress
│  │  ├─ compress_challenge.ipynb
│  │  ├─ compress_solution.ipynb
│  │  ├─ test_compress.py
│  ├─ ...
├─ linked_lists
│  ├─ palindrome
│  │  └─ ...
│  ├─ ...
├─ ...

*The notebooks (.ipynb) read/write the associated unit test (.py) file.

Notebook Installation

Zero Install

This README contains links to Binder , which hosts dynamic notebooks of the repo’s contents online with no installation needed.

Jupyter Notebook

Run:

pip install jupyter

For detailed instructions, scripts, and tools to more optimally set up your development environment, check out the dev-setup repo.

For more details on notebook installation, follow the directions here.

More information on IPython/Jupyter Notebooks can be found here.

Running Challenges

Notebooks

Challenges are provided in the form of IPython/Jupyter Notebooks and have been tested with Python 2.7 and Python 3.x.

If you need to install IPython/Jupyter Notebook, see the Notebook Installation section.

This README contains links to nbviewer, which hosts static notebooks of the repo’s contents
To interact with or to modify elements within the dynamic notebooks, refer to the instructions below

Run the notebook of challenges:

$ git clone https://github.com/donnemartin/interactive-coding-challenges.git
$ cd interactive-coding-challenges
$ jupyter notebook

This will launch your web browser with the list of challenge categories:

Navigate to the Challenge Notebook you wish to solve
Run the cells within the challenge notebook (Cell->Run All)
- This will result in an expected unit test error
Solve the challenge and verify it passes the unit test
Check out the accompanying Solution Notebook for further discussion

To debug your solution with pdb, refer to the following ticket.

Note: If your solution is different from those listed in the Solution Notebook, consider submitting a pull request so others can benefit from your work. Review the Contributing Guidelines for details.

Future Development

Challenges, solutions, and unit tests are presented in the form of IPython/Jupyter Notebooks.

Notebooks currently contain mostly Python solutions (tested on both Python 2.7 and Python 3.x), but can be extended to include 40+ supported languages
Repo will be continually updated with new solutions and challenges
Contributions are welcome!

Contributing

Contributions are welcome!

Review the Contributing Guidelines for details on how to:

Submit issues
Add solutions to existing challenges
Add new challenges

Credits

Resources

Images

Contact Info

Feel free to contact me to discuss any issues, questions, or comments.

My contact info can be found on my GitHub page.

License

I am providing code and resources in this repository to you under an open source license. Because this is my personal repository, the license you receive to my code and resources is from me and not my employer (Facebook).

Copyright 2015 Donne Martin

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

   http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.