Intro

Welcome to the Hackbright Code Challenges book. This book collects problems that we think are appropriate for alumnae of our program that will test and reinforce the concepts taught in the Fellowship, as well as adding a few new advanced concepts.

This guide was primarily written by Joel Burton with contributions from Jennifer Griffith-Delgado, Agnes Klimaite, Henry Chen, Ahmad Alawad, Bonnie Schulkin, Meggie Mahnken, Sarah Stringer, and Lavinia Karl.

It is dedicated to all Hackbright Teaching Assistants, past and present.

We provide two kinds of challenges:

Whiteboard Problems

These are shorter, simpler problems designed to be done live at a whiteboard.

While it’s alway hard to guess how difficult a problem will be, our general rubric for whiteboard problems is:

Easier: 90% of alumnae should be able to solve in under 15 minutes.
Medium: 80% of alumnae should be able to solve in under 25 minutes.
Harder: 50% of alumnae should be able to solve in under 25 minutes.

Code Challenges

These are problems to worked on solo or in groups over a period of time, using computers and being able to run/test their code as they go.

Our estimated difficulty for these are:

Easy: 90% of alumnae should be able to solve in less than one hour.
Medium: 80% of alumnae should be able to solve in less than three hours.
Hard: 50% of alumnae should be able to solve in a day. (Some of these are very tricky, and would be exceptionally hard problems for Junior Developer interviews!)

All Problems

Add Linked Lists: Given two linked lists, treat as numbers and add them together.

Whiteboard Level: None

Coding Challenge Level: Harder

Concepts: Linked Lists
Add To Zero: Given list of ints, return True if any two nums in list sum to 0.

Whiteboard Level: Easier

Coding Challenge Level: None

Concepts: Loops, General
Anagram of Palindrome: Is a word an anagram of a palindrome?

Whiteboard Level: Medium

Coding Challenge Level: Easier

Concepts: Dictionaries, General
BST Add Child: Insert a new node into right place in a binary search tree.

Whiteboard Level: Harder

Coding Challenge Level: Medium

Concepts: BST, Recursion
Balanced Brackets: Does a given string have balanced pairs of brackets?

Whiteboard Level: Harder

Coding Challenge Level: Medium

Concepts: Conditionals, Loops
Balanced Parentheses: Given a string, check whether its parentheses are balanced.

Whiteboard Level: Medium

Coding Challenge Level: None

Concepts: Conditionals, Loops
Battleship!: Finish an incomplete version of the game Battleship.

Whiteboard Level: None

Coding Challenge Level: Harder

Concepts: Object Orientation, Reading Code
Binary Search: Write an optimal number guessing user binary search.

Whiteboard Level: Medium

Coding Challenge Level: Easier

Concepts: Binary Search
Binary Search Tree Validator: Check if a BST is valid.

Whiteboard Level: Harder

Coding Challenge Level: Medium

Concepts: Recursion, Trees
Boggle: Check if a word is on a Boggle board.

Whiteboard Level: None

Coding Challenge Level: Harder

Concepts: Recursion
Calculator: Build a “polish notation” calculator.

Whiteboard Level: Harder

Coding Challenge Level: Medium

Concepts: Stacks
Check Detection: Determine if a chess player is in check.

Whiteboard Level: Harder

Coding Challenge Level: Medium

Concepts: Data Structures, General
Check Tree Balance: Is this tree balanced?

Whiteboard Level: None

Coding Challenge Level: Medium

Concepts: Trees, Recursion
Circular Array: Build a circular array, a useful data structure.

Whiteboard Level: None

Coding Challenge Level: Medium

Concepts: Lists, Data Structures
Coins: Finding possibilities for making change using recursion.

Whiteboard Level: Harder

Coding Challenge Level: Medium

Concepts: Recursion
Compress a String: Compress a string with a simple compression algorithm

Whiteboard Level: Harder

Coding Challenge Level: Easier

Concepts: Strings, Loops
Concatenate Lists: Concatenate one list at the end of another.

Whiteboard Level: Easier

Coding Challenge Level: None

Concepts: Lists
Count Employees: Find number of employees in an organization chart.

Whiteboard Level: Harder

Coding Challenge Level: Medium

Concepts: Recursion, Trees
Count List Recursively: Count the number of items in a list using recursion.

Whiteboard Level: Medium

Coding Challenge Level: Easier

Concepts: Recursion
Days in Month: Given a month and a year, return how many days are in that month.

Whiteboard Level: Easier

Coding Challenge Level: None

Concepts: Conditionals, Loops
Decimal To Binary: Convert a decimal number to binary representation.

Whiteboard Level: Harder

Coding Challenge Level: Medium

Concepts: General
Decode a String: Decode a string into the original text.

Whiteboard Level: Medium

Coding Challenge Level: Easier

Concepts: Loops
Encipher a String: Encode a string using a variable Caesar Cipher

Whiteboard Level: Medium

Coding Challenge Level: Medium

Concepts: Strings, General
Find Lucky Numbers: Return n unique random numbers from a list.

Whiteboard Level: Easier

Coding Challenge Level: None

Concepts: Loops, General
Find the Range: Find the smallest and largest number in a list.

Whiteboard Level: Easier

Coding Challenge Level: None

Concepts: Lists, Loops
Fit String to Width: Print a string within a character limit

Whiteboard Level: Medium

Coding Challenge Level: Easier

Concepts: Loops
FizzBuzz: Print a series of messages of “fizz”, “buzz”, and “fizzbuzz”.

Whiteboard Level: Easier

Coding Challenge Level: None

Concepts: Loops
Friends: Find out if two friends in a graph are connected.

Whiteboard Level: Harder

Coding Challenge Level: Medium

Concepts: Graphs, Recursion
Has More Vowels: Given a word, return True if it has more vowels than consonants.

Whiteboard Level: Easier

Coding Challenge Level: None

Concepts: Loops, Conditionals
Has Unique Characters: Given a word, return True if it contains no duplicate characters.

Whiteboard Level: Easier

Coding Challenge Level: None

Concepts: Sets, Conditions
Hexadecimal Conversion: Convert a number from hexadecimal to decimal.

Whiteboard Level: Harder

Coding Challenge Level: Medium

Concepts: Math, General
Insertion Sort: Implement insertion sort, a straightforward sorting algorithm.

Whiteboard Level: Harder

Coding Challenge Level: Medium

Concepts: Sorting
Is Number Prime?: Is a number a prime number?

Whiteboard Level: Easier

Coding Challenge Level: None

Concepts: General, Math
Is Palindrome: Is a word a palindrome?

Whiteboard Level: Easier

Coding Challenge Level: None

Concepts: Loops, General
Josephus Survivors: Determine who will survive in a circle.

Whiteboard Level: Harder

Coding Challenge Level: Medium

Concepts: General, Linked Lists
Largest Smaller Than: Find largest number in nums list smaller than x.

Whiteboard Level: Easier

Coding Challenge Level: None

Concepts: Loops, Lists, Conditionals
Largest Subsequence Sum: Find the highest subsequence sum in a list.

Whiteboard Level: Harder

Coding Challenge Level: Medium

Concepts: General
Lazy Lemmings: Help our lemmings find the nearest cafe.

Whiteboard Level: Medium

Coding Challenge Level: Easier

Concepts: Loops, List, Binary Search
Leaping Lemur: Help our sprightly lemur cross the forest.

Whiteboard Level: Medium

Coding Challenge Level: Easier

Concepts: Loops, Lists
Leet Speak: Translate a string into leet speak.

Whiteboard Level: Easier

Coding Challenge Level: None

Concepts: Strings
Leveret Lunch: Help our hungry leveret find her lunch.

Whiteboard Level: Harder

Coding Challenge Level: Medium

Concepts: Loops, Lists
Longest Word: Find the longest word in a list.

Whiteboard Level: Easier

Coding Challenge Level: None

Concepts: Strings, Lists
Make Binary Search Tree: Make a balanced binary search from a list.

Whiteboard Level: None

Coding Challenge Level: Medium

Concepts: Trees, Recursion
Max Number: Implementation of max.

Whiteboard Level: Easier

Coding Challenge Level: None

Concepts: Math, Loops, Conditionals
Max Path of Triangle: What’s the high-scoring path for navigating a DAG?

Whiteboard Level: Harder

Coding Challenge Level: Medium

Concepts: Trees, Graphs
Max of Three: Given 3 integers, return the largest one.

Whiteboard Level: Easier

Coding Challenge Level: None

Concepts: Math, Conditionals
Merge Sort: Implement merge sort, a high-performance sorting algorithm.

Whiteboard Level: Harder

Coding Challenge Level: Medium

Concepts: Sorting
Minesweeper!: Complete an incomplete version of Minesweeper.

Whiteboard Level: None

Coding Challenge Level: Harder

Concepts: Object Orientation, Reading Code
Missing Number: Find the missing number in a list.

Whiteboard Level: Medium

Coding Challenge Level: Easier

Concepts: Runtime, General
Mode: Find the most frequent number in a list.

Whiteboard Level: Medium

Coding Challenge Level: None

Concepts: Lists, Loops, Math
Monkey River Crossing: Help the monkey cross the river by jumping on stones.

Whiteboard Level: Harder

Coding Challenge Level: Medium

Concepts: Logic, Lists
Most Active Period: Find the window of time when most authors were active.

Whiteboard Level: Harder

Coding Challenge Level: Medium

Concepts: Logic, Loops
Most Common Anagram: Find the most common anagram in a list of words.

Whiteboard Level: None

Coding Challenge Level: Medium

Concepts: General
Number To Word: Convert a number into words.

Whiteboard Level: None

Coding Challenge Level: Medium

Concepts: General
One Edit Away?: Are two words, at most, one edit away from each other?

Whiteboard Level: None

Coding Challenge Level: Medium

Concepts: Strings, Logic
Pangram: Return True if a sentence contains all the letters of alphabet.

Whiteboard Level: Easier

Coding Challenge Level: Easier

Concepts: Sets, Strings
Pattern Matching: Can a string match a given pattern?

Whiteboard Level: None

Coding Challenge Level: Harder

Concepts: Algorithm Design
Pig Latin: Translate text into Pig Latin.

Whiteboard Level: Easier

Coding Challenge Level: Easier

Concepts: General
Pivot Linked List: Pivot a linked list around a particular value.

Whiteboard Level: Harder

Coding Challenge Level: Medium

Concepts: Logic, Linked Lists
Poker: Evaluate a poker hand for strength.

Whiteboard Level: None

Coding Challenge Level: Harder

Concepts: Logic, Data Structures
Prime Number Generator: Generate a certain number of prime numbers.

Whiteboard Level: Harder

Coding Challenge Level: Easier

Concepts: Math, General
Print Digits Backwards: Print digits of a number on backwards order.

Whiteboard Level: Medium

Coding Challenge Level: Easier

Concepts: General, Math
Print List Recursively: Print items in a list using recursion.

Whiteboard Level: Medium

Coding Challenge Level: Easier

Concepts: Recursion
Rain in Codelandia: How much rain is gathered on buildings?

Whiteboard Level: None

Coding Challenge Level: Harder

Concepts: Logic, Loops
Recursive Index: Search a list using recursion.

Whiteboard Level: Medium

Coding Challenge Level: Easier

Concepts: Recursion
Remove Duplicates: Remove duplicates in an a list and keep order.

Whiteboard Level: Easier

Coding Challenge Level: Easier

Concepts: Lists, Sets
Remove Linked List Node: Remove a node from the start/middle of a linked list.

Whiteboard Level: Medium

Coding Challenge Level: Easier

Concepts: Linked Lists
Replace Vowels: Replace all vowels in a list of chars with star.

Whiteboard Level: Easier

Coding Challenge Level: None

Concepts: Loops, Lists
Reverse Linked List: Reverse a linked list, returning a new list.

Whiteboard Level: Medium

Coding Challenge Level: Easier

Concepts: Linked Lists
Reverse Linked List In Place: Reverse a linked list working in place.

Whiteboard Level: Harder

Coding Challenge Level: Easier

Concepts: Linked Lists
Reverse List in Place: Reverse a list, working in place.

Whiteboard Level: Easier

Coding Challenge Level: None

Concepts: Loops, Lists
Reverse Words: Reverse words in a string, preserving spaces.

Whiteboard Level: Harder

Coding Challenge Level: Medium

Concepts: Strings, Loops
Reverse a String: Reverse characters in a string without using reversed.

Whiteboard Level: Easier

Coding Challenge Level: None

Concepts: Loops, Strings
Reverse a String Recursively: Reverse characters in a string using recursion.

Whiteboard Level: Medium

Coding Challenge Level: Easier

Concepts: Recursion, Strings
Roman Numerals: Convert a decimal number into roman numerals.

Whiteboard Level: Harder

Coding Challenge Level: Medium

Concepts: General, Math
SQL Managers: Design and query a database for an organization chart.

Whiteboard Level: None

Coding Challenge Level: Medium

Concepts: SQL, Joins, Self-Joins
Same Line: Find points on lines that have 3+ points on it.

Whiteboard Level: Harder

Coding Challenge Level: Medium

Concepts: Loops, Geometry
Show Even Numbers: Show indexes of even numbers in a list.

Whiteboard Level: Easier

Coding Challenge Level: Easier

Concepts: Loops
Simple Infix Calculator: Write a calculator for simple math expressions.

Whiteboard Level: None

Coding Challenge Level: Harder

Concepts: Data Structures
Smallest Difference: Find smallest difference between numbers in lists.

Whiteboard Level: None

Coding Challenge Level: Harder

Concepts: Algorithms, Runtime
Snake Case to Camel Case: Convert string from snake_case to camelCase.

Whiteboard Level: Easier

Coding Challenge Level: None

Concepts: Strings
Sort Sorted Lists: Merge together two already-sorted lists.

Whiteboard Level: Medium

Coding Challenge Level: Easier

Concepts: Loops, Sorting
Spiral: Move in a spiral direction around a matrix.

Whiteboard Level: None

Coding Challenge Level: Harder

Concepts: Data Structures
Split Square: Design data and recursive algorithms for “split squares”.

Whiteboard Level: None

Coding Challenge Level: Harder

Concepts: Recursion, Data Structures
Split a String: Split a string on another, like the Python built-in split.

Whiteboard Level: Medium

Coding Challenge Level: Easier

Concepts: Loops, Strings
Staircase Steps: How many ways are there to climb a staircase?

Whiteboard Level: None

Coding Challenge Level: Harder

Concepts: Recursion, Dynamic Programming
Stock Prices: Find the maximum possible stock profit for a day.

Whiteboard Level: Harder

Coding Challenge Level: Medium

Concepts: Logic
Sum List: Implementation of sum.

Whiteboard Level: Easier

Coding Challenge Level: None

Concepts: Math, Loops
Sum List Recursively: Implement sum using recursion.

Whiteboard Level: Medium

Coding Challenge Level: Easier

Concepts: Recursion, Lists
Takeaway: Write an AI for a simple game.

Whiteboard Level: Harder

Coding Challenge Level: Medium

Concepts: Game Theory
Tic-Tac-Random: Complete a tic-tac-toe game with a random AI.

Whiteboard Level: None

Coding Challenge Level: Harder

Concepts: Data Structures
Time Word: Convert a time into words.

Whiteboard Level: None

Coding Challenge Level: Medium

Concepts: General
Towers of Hanoi: Solve a classic recursion problem involving stacks of disks.

Whiteboard Level: None

Coding Challenge Level: Harder

Concepts: Recursion, Stacks
Tree Cousins: Find nodes at the same level as current node.

Whiteboard Level: Harder

Coding Challenge Level: Medium

Concepts: Trees
Truncate Repeats: Truncate repeating characters in a string

Whiteboard Level: Easier

Coding Challenge Level: Easier

Concepts: Strings, Loops
Word Break: Find ways of breaking string into known words.

Whiteboard Level: Harder

Coding Challenge Level: Medium

Concepts: Recursion
Word Count: Count words in a sentence, and print in ascending order.

Whiteboard Level: Easier

Coding Challenge Level: None

Concepts: Strings, Dictionaries
Word Lengths: Find the lengths of words in a string.

Whiteboard Level: Easier

Coding Challenge Level: None

Concepts: Dictionaries, Loops
Word Prefixes and Tries: Find words with matching prefixes using trie.

Whiteboard Level: None

Coding Challenge Level: Harder

Concepts: Trees, Tries
Zero Out Matrix: Update a matrix to zero out some columns and rows.

Whiteboard Level: None

Coding Challenge Level: Medium

Concepts: Data Structures

Whiteboarding Problems

Easier

Add To Zero: Given list of ints, return True if any two nums in list sum to 0.

Concepts: Loops, General
Concatenate Lists: Concatenate one list at the end of another.

Concepts: Lists
Days in Month: Given a month and a year, return how many days are in that month.

Concepts: Conditionals, Loops
Find Lucky Numbers: Return n unique random numbers from a list.

Concepts: Loops, General
Find the Range: Find the smallest and largest number in a list.

Concepts: Lists, Loops
FizzBuzz: Print a series of messages of “fizz”, “buzz”, and “fizzbuzz”.

Concepts: Loops
Has More Vowels: Given a word, return True if it has more vowels than consonants.

Concepts: Loops, Conditionals
Has Unique Characters: Given a word, return True if it contains no duplicate characters.

Concepts: Sets, Conditions
Is Number Prime?: Is a number a prime number?

Concepts: General, Math
Is Palindrome: Is a word a palindrome?

Concepts: Loops, General
Largest Smaller Than: Find largest number in nums list smaller than x.

Concepts: Loops, Lists, Conditionals
Leet Speak: Translate a string into leet speak.

Concepts: Strings
Longest Word: Find the longest word in a list.

Concepts: Strings, Lists
Max Number: Implementation of max.

Concepts: Math, Loops, Conditionals
Max of Three: Given 3 integers, return the largest one.

Concepts: Math, Conditionals
Pangram: Return True if a sentence contains all the letters of alphabet.

Concepts: Sets, Strings
Pig Latin: Translate text into Pig Latin.

Concepts: General
Remove Duplicates: Remove duplicates in an a list and keep order.

Concepts: Lists, Sets
Replace Vowels: Replace all vowels in a list of chars with star.

Concepts: Loops, Lists
Reverse List in Place: Reverse a list, working in place.

Concepts: Loops, Lists
Reverse a String: Reverse characters in a string without using reversed.

Concepts: Loops, Strings
Show Even Numbers: Show indexes of even numbers in a list.

Concepts: Loops
Snake Case to Camel Case: Convert string from snake_case to camelCase.

Concepts: Strings
Sum List: Implementation of sum.

Concepts: Math, Loops
Truncate Repeats: Truncate repeating characters in a string

Concepts: Strings, Loops
Word Count: Count words in a sentence, and print in ascending order.

Concepts: Strings, Dictionaries
Word Lengths: Find the lengths of words in a string.

Concepts: Dictionaries, Loops

Medium

Anagram of Palindrome: Is a word an anagram of a palindrome?

Concepts: Dictionaries, General
Balanced Parentheses: Given a string, check whether its parentheses are balanced.

Concepts: Conditionals, Loops
Binary Search: Write an optimal number guessing user binary search.

Concepts: Binary Search
Count List Recursively: Count the number of items in a list using recursion.

Concepts: Recursion
Decode a String: Decode a string into the original text.

Concepts: Loops
Encipher a String: Encode a string using a variable Caesar Cipher

Concepts: Strings, General
Fit String to Width: Print a string within a character limit

Concepts: Loops
Lazy Lemmings: Help our lemmings find the nearest cafe.

Concepts: Loops, List, Binary Search
Leaping Lemur: Help our sprightly lemur cross the forest.

Concepts: Loops, Lists
Missing Number: Find the missing number in a list.

Concepts: Runtime, General
Mode: Find the most frequent number in a list.

Concepts: Lists, Loops, Math
Print Digits Backwards: Print digits of a number on backwards order.

Concepts: General, Math
Print List Recursively: Print items in a list using recursion.

Concepts: Recursion
Recursive Index: Search a list using recursion.

Concepts: Recursion
Remove Linked List Node: Remove a node from the start/middle of a linked list.

Concepts: Linked Lists
Reverse Linked List: Reverse a linked list, returning a new list.

Concepts: Linked Lists
Reverse a String Recursively: Reverse characters in a string using recursion.

Concepts: Recursion, Strings
Sort Sorted Lists: Merge together two already-sorted lists.

Concepts: Loops, Sorting
Split a String: Split a string on another, like the Python built-in split.

Concepts: Loops, Strings
Sum List Recursively: Implement sum using recursion.

Concepts: Recursion, Lists

Harder

BST Add Child: Insert a new node into right place in a binary search tree.

Concepts: BST, Recursion
Balanced Brackets: Does a given string have balanced pairs of brackets?

Concepts: Conditionals, Loops
Binary Search Tree Validator: Check if a BST is valid.

Concepts: Recursion, Trees
Calculator: Build a “polish notation” calculator.

Concepts: Stacks
Check Detection: Determine if a chess player is in check.

Concepts: Data Structures, General
Coins: Finding possibilities for making change using recursion.

Concepts: Recursion
Compress a String: Compress a string with a simple compression algorithm

Concepts: Strings, Loops
Count Employees: Find number of employees in an organization chart.

Concepts: Recursion, Trees
Decimal To Binary: Convert a decimal number to binary representation.

Concepts: General
Friends: Find out if two friends in a graph are connected.

Concepts: Graphs, Recursion
Hexadecimal Conversion: Convert a number from hexadecimal to decimal.

Concepts: Math, General
Insertion Sort: Implement insertion sort, a straightforward sorting algorithm.

Concepts: Sorting
Josephus Survivors: Determine who will survive in a circle.

Concepts: General, Linked Lists
Largest Subsequence Sum: Find the highest subsequence sum in a list.

Concepts: General
Leveret Lunch: Help our hungry leveret find her lunch.

Concepts: Loops, Lists
Max Path of Triangle: What’s the high-scoring path for navigating a DAG?

Concepts: Trees, Graphs
Merge Sort: Implement merge sort, a high-performance sorting algorithm.

Concepts: Sorting
Monkey River Crossing: Help the monkey cross the river by jumping on stones.

Concepts: Logic, Lists
Most Active Period: Find the window of time when most authors were active.

Concepts: Logic, Loops
Pivot Linked List: Pivot a linked list around a particular value.

Concepts: Logic, Linked Lists
Prime Number Generator: Generate a certain number of prime numbers.

Concepts: Math, General
Reverse Linked List In Place: Reverse a linked list working in place.

Concepts: Linked Lists
Reverse Words: Reverse words in a string, preserving spaces.

Concepts: Strings, Loops
Roman Numerals: Convert a decimal number into roman numerals.

Concepts: General, Math
Same Line: Find points on lines that have 3+ points on it.

Concepts: Loops, Geometry
Stock Prices: Find the maximum possible stock profit for a day.

Concepts: Logic
Takeaway: Write an AI for a simple game.

Concepts: Game Theory
Tree Cousins: Find nodes at the same level as current node.

Concepts: Trees
Word Break: Find ways of breaking string into known words.

Concepts: Recursion

Coding Challenge Problems

Easier

Anagram of Palindrome: Is a word an anagram of a palindrome?

Concepts: Dictionaries, General
Binary Search: Write an optimal number guessing user binary search.

Concepts: Binary Search
Compress a String: Compress a string with a simple compression algorithm

Concepts: Strings, Loops
Count List Recursively: Count the number of items in a list using recursion.

Concepts: Recursion
Decode a String: Decode a string into the original text.

Concepts: Loops
Fit String to Width: Print a string within a character limit

Concepts: Loops
Lazy Lemmings: Help our lemmings find the nearest cafe.

Concepts: Loops, List, Binary Search
Leaping Lemur: Help our sprightly lemur cross the forest.

Concepts: Loops, Lists
Missing Number: Find the missing number in a list.

Concepts: Runtime, General
Pangram: Return True if a sentence contains all the letters of alphabet.

Concepts: Sets, Strings
Pig Latin: Translate text into Pig Latin.

Concepts: General
Prime Number Generator: Generate a certain number of prime numbers.

Concepts: Math, General
Print Digits Backwards: Print digits of a number on backwards order.

Concepts: General, Math
Print List Recursively: Print items in a list using recursion.

Concepts: Recursion
Recursive Index: Search a list using recursion.

Concepts: Recursion
Remove Duplicates: Remove duplicates in an a list and keep order.

Concepts: Lists, Sets
Remove Linked List Node: Remove a node from the start/middle of a linked list.

Concepts: Linked Lists
Reverse Linked List: Reverse a linked list, returning a new list.

Concepts: Linked Lists
Reverse Linked List In Place: Reverse a linked list working in place.

Concepts: Linked Lists
Reverse a String Recursively: Reverse characters in a string using recursion.

Concepts: Recursion, Strings
Show Even Numbers: Show indexes of even numbers in a list.

Concepts: Loops
Sort Sorted Lists: Merge together two already-sorted lists.

Concepts: Loops, Sorting
Split a String: Split a string on another, like the Python built-in split.

Concepts: Loops, Strings
Sum List Recursively: Implement sum using recursion.

Concepts: Recursion, Lists
Truncate Repeats: Truncate repeating characters in a string

Concepts: Strings, Loops

Medium

BST Add Child: Insert a new node into right place in a binary search tree.

Concepts: BST, Recursion
Balanced Brackets: Does a given string have balanced pairs of brackets?

Concepts: Conditionals, Loops
Binary Search Tree Validator: Check if a BST is valid.

Concepts: Recursion, Trees
Calculator: Build a “polish notation” calculator.

Concepts: Stacks
Check Detection: Determine if a chess player is in check.

Concepts: Data Structures, General
Check Tree Balance: Is this tree balanced?

Concepts: Trees, Recursion
Circular Array: Build a circular array, a useful data structure.

Concepts: Lists, Data Structures
Coins: Finding possibilities for making change using recursion.

Concepts: Recursion
Count Employees: Find number of employees in an organization chart.

Concepts: Recursion, Trees
Decimal To Binary: Convert a decimal number to binary representation.

Concepts: General
Encipher a String: Encode a string using a variable Caesar Cipher

Concepts: Strings, General
Friends: Find out if two friends in a graph are connected.

Concepts: Graphs, Recursion
Hexadecimal Conversion: Convert a number from hexadecimal to decimal.

Concepts: Math, General
Insertion Sort: Implement insertion sort, a straightforward sorting algorithm.

Concepts: Sorting
Josephus Survivors: Determine who will survive in a circle.

Concepts: General, Linked Lists
Largest Subsequence Sum: Find the highest subsequence sum in a list.

Concepts: General
Leveret Lunch: Help our hungry leveret find her lunch.

Concepts: Loops, Lists
Make Binary Search Tree: Make a balanced binary search from a list.

Concepts: Trees, Recursion
Max Path of Triangle: What’s the high-scoring path for navigating a DAG?

Concepts: Trees, Graphs
Merge Sort: Implement merge sort, a high-performance sorting algorithm.

Concepts: Sorting
Monkey River Crossing: Help the monkey cross the river by jumping on stones.

Concepts: Logic, Lists
Most Active Period: Find the window of time when most authors were active.

Concepts: Logic, Loops
Most Common Anagram: Find the most common anagram in a list of words.

Concepts: General
Number To Word: Convert a number into words.

Concepts: General
One Edit Away?: Are two words, at most, one edit away from each other?

Concepts: Strings, Logic
Pivot Linked List: Pivot a linked list around a particular value.

Concepts: Logic, Linked Lists
Reverse Words: Reverse words in a string, preserving spaces.

Concepts: Strings, Loops
Roman Numerals: Convert a decimal number into roman numerals.

Concepts: General, Math
SQL Managers: Design and query a database for an organization chart.

Concepts: SQL, Joins, Self-Joins
Same Line: Find points on lines that have 3+ points on it.

Concepts: Loops, Geometry
Stock Prices: Find the maximum possible stock profit for a day.

Concepts: Logic
Takeaway: Write an AI for a simple game.

Concepts: Game Theory
Time Word: Convert a time into words.

Concepts: General
Tree Cousins: Find nodes at the same level as current node.

Concepts: Trees
Word Break: Find ways of breaking string into known words.

Concepts: Recursion
Zero Out Matrix: Update a matrix to zero out some columns and rows.

Concepts: Data Structures

Harder

Add Linked Lists: Given two linked lists, treat as numbers and add them together.

Concepts: Linked Lists
Battleship!: Finish an incomplete version of the game Battleship.

Concepts: Object Orientation, Reading Code
Boggle: Check if a word is on a Boggle board.

Concepts: Recursion
Minesweeper!: Complete an incomplete version of Minesweeper.

Concepts: Object Orientation, Reading Code
Pattern Matching: Can a string match a given pattern?

Concepts: Algorithm Design
Poker: Evaluate a poker hand for strength.

Concepts: Logic, Data Structures
Rain in Codelandia: How much rain is gathered on buildings?

Concepts: Logic, Loops
Simple Infix Calculator: Write a calculator for simple math expressions.

Concepts: Data Structures
Smallest Difference: Find smallest difference between numbers in lists.

Concepts: Algorithms, Runtime
Spiral: Move in a spiral direction around a matrix.

Concepts: Data Structures
Split Square: Design data and recursive algorithms for “split squares”.

Concepts: Recursion, Data Structures
Staircase Steps: How many ways are there to climb a staircase?

Concepts: Recursion, Dynamic Programming
Tic-Tac-Random: Complete a tic-tac-toe game with a random AI.

Concepts: Data Structures
Towers of Hanoi: Solve a classic recursion problem involving stacks of disks.

Concepts: Recursion, Stacks
Word Prefixes and Tries: Find words with matching prefixes using trie.

Concepts: Trees, Tries