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Free C# Course: Zero to Interview-Ready

Learn C# from your first line of code to the patterns coding interviews ask about. Every lesson is interactive: write the code in your browser and check each step as you go. Try the first step of any lesson free, no sign-up needed.

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PART 1

Learn to code

Coding fundamentals, from your first line of code to solving problems on your own.

Welcome To C#

  1. Printing to the Console
  2. Intro to Strings
  3. Comments
  4. Intro to Variables
  5. Syntax Errors
  6. Name Errors
  7. Strings Vs Numbers
  8. String Concatenation

Variables

  1. The Power Of Variables
  2. Data Types
  3. Variable Naming Rules
  4. Variable Reassignment
  5. Constants
  6. Type Conversion

Calculations With Numbers

  1. Arithmetic Operators
  2. Area Of Rectangle
  3. Mathematical Expressions
  4. Find Discount
  5. Operator Precedence
  6. Price After Discount
  7. Convert Hours And Mins Into Seconds
  8. Floor Division
  9. Free Coffee Cups
  10. Modulo Operator
  11. Augmented Assignment Operators
  12. Increment And Decrement Operators

Functions I

  1. Functions
  2. Defining Functions: Buggy Code
  3. Defining Functions: Quiz
  4. Calling Functions: Buggy Code
  5. Calling Functions: Quiz
  6. Execution Flow: Quiz
  7. Why Functions
  8. Exercise: Create a Function
  9. Function Parameters & Arguments
  10. Parameters & Arguments: Buggy Code I
  11. Parameters & Arguments: Buggy Code II
  12. Why Parameters & Arguments
  13. Exercise: Create Function With Parameters
  14. Returning From Functions
  15. Returning From Functions: Quiz I
  16. Returning From Functions: Buggy Code I
  17. Returning From Functions: Quiz II
  18. Returning From Functions: Buggy Code II
  19. Returning From Functions: Quiz III
  20. Returning From Functions: Buggy Code III
  21. Returning From Functions: Quiz IV
  22. Returning From Functions: Buggy Code IV

Calculations - Challenges

  1. Your First Challenge
  2. Buy Candy
  3. Hand Shakes
  4. Last Two Digit Sum

Strings I

  1. Escaping Characters
  2. String Concatenation II
  3. Accessing Characters
  4. String Slicing
  5. String Immutability
  6. String Length
  7. Exceeding String Bounds
  8. String Find
  9. String Replace
  10. String Split
  11. String Strip / Trim
  12. Uppercase and Lowercase

Conditional Statements

  1. If Statements
  2. If-Else Statements
  3. Comparison Operators
  4. Is Number Negative
  5. Student Passed
  6. Return Negative
  7. Redundant Else Statement
  8. Parity of Number
  9. And Operator
  10. Reducing If Else To Boolean Expression
  11. Allow to Contest
  12. Or Operator
  13. Not Operator
  14. Combining Logical Operators
  15. Chaining Conditional Statements
  16. Nullable Value Types

Conditional Statements - Challenges

  1. Recreating ABS
  2. Recreating Absolute Difference
  3. Leap Year
  4. Interval Intersection
  5. Second Largest Value

Arrays I

  1. Static Arrays
  2. ArrayLists
  3. Accessing Static Array Elements
  4. Accessing ArrayLists Elements
  5. Updating Static Array Elements
  6. Updating ArrayLists Elements
  7. Static Array Length
  8. ArrayLists Size
  9. Exceeding Static Array Bounds
  10. Exceeding ArrayLists bounds
  11. Append Item to Array
  12. Remove Last Item from Array
  13. LINQ: Where & Select

Dictionaries

  1. Dictionaries (HashMap)
  2. Accessing Dictionary Values
  3. Updating Dictionary Values
  4. Dictionary Length

For Loops - Looping Over Arrays

  1. Why For Loops?
  2. For Loop - Looping over Arrays
  3. Looping over Arrays: Buggy Code I
  4. Looping over Arrays: Buggy Code II
  5. For Loop - Execution Flow
  6. For Loop - Execution Flow: Buggy Code I
  7. For Loop - Execution Flow: Buggy Code II
  8. For Loop - Execution Flow: Buggy Code III
  9. Infinite For Loops

While Loops & For Loops

  1. Why Loops?
  2. The While Loop
  3. While Loop: Initialization & Condition
  4. While Loop: Iteration
  5. Infinite While Loops
  6. The Classic For Loop
  7. For Loop: Initialization & Condition
  8. For Loop: Iteration
  9. Looping in Reverse
  10. Infinite For Loops II
  11. Print Multiples

Loops & Conditionals

  1. Positive Numbers From Array
  2. Even Numbers From Array: Buggy Code
  3. Print Even-Odd Numbers
  4. Print Odd Numbers: Buggy Code
  5. Print Positive-Negative Numbers: Buggy Code
  6. Array Contains Number
  7. Positive Number in Array: Buggy Code
  8. Hurdle Jump
  9. Prime Number
  10. Collatz Conjecture

Loops & Global Variables

  1. Sum of Numbers
  2. Sum of Numbers: Buggy Code
  3. Sum of Squares
  4. Factorial
  5. Array Sum: Buggy Code
  6. Student Grades
  7. Number Of Occurences In Array
  8. H4ck3r Sp34k
  9. Maximum Number In Array
  10. Check If Odd > Even

Looping Over Array Indices

  1. Looping Through Arrays With Indices
  2. Number Index In Array
  3. Lower Bound Index
  4. Manipulating Array Elements inside Loop
  5. Get Full Names I
  6. Reverse Array
  7. Break In For Loops
  8. Continue In For Loops

C# Essentials

  1. Nullable Value Types
  2. LINQ: Where & Select
  3. Properties

PART 2

Level up

Big O, strings, classes and object-oriented programming: write bigger programs and know how fast they run.

Big O Notation

  1. Linear Time Complexity
  2. Constant Time Complexity
  3. Built-in Functions - Time Complexity
  4. Array & String Methods - Time Complexity
  5. Slicing & Concatenation - Time Complexity
  6. Quadratic Time Complexity
  7. Space Complexity

Loops - Challenges

  1. Print Powers
  2. Fizz Buzz
  3. Longest Common Prefix Of Two Strings
  4. Get Full Names II
  5. Find Largest Number
  6. Fibonacci
  7. Tribonacci
  8. Max Val And Number Of Occurrences

Strings

  1. Intro To Strings
  2. Strings
  3. Reverse String II
  4. Reverse String III
  5. Palindrome Check II
  6. Reverse Words
  7. Print Triangle Of Stars

Functions & Classes

  1. Classes
  2. Magical Number
  3. Print Rhombus
  4. Print X

Object-Oriented Programming

  1. Defining a Class
  2. Creating Objects
  3. Constructors
  4. Class Fields
  5. Class Methods
  6. Access Modifiers
  7. Inheritance
  8. Method Overriding
  9. Polymorphism
  10. Abstract Classes
  11. Interfaces
  12. OOP Capstone
  13. Properties

PART 3

Coding interviews

From brute force to dynamic programming: the patterns behind the questions top tech companies ask.

Free Coding Interview Questions

  1. Minimum Value of Three
  2. Max Val And Number Of Occurrences
  3. Maximum Sum Subarray
  4. Number Of Islands
  5. Binary Strings With At Most K Consecutive Ones

Nested Loops & Brute Force

  1. Nested Loops
  2. Pair Count
  3. Two Sum
  4. Selection Sort
  5. Remove Duplicates From Array
  6. Longest Subarray Without Reapeating
  7. Minimum Absolute Difference

Sorting

  1. Sorting
  2. Smallest K
  3. Distinct Values
  4. Remove Duplicates From Array II
  5. Longest Consecutive Sequence
  6. Group Anagrams
  7. Find Second Largest
  8. Minimum Absolute Difference II

Partial Sums #1

  1. Partial Sums
  2. Maximum Sum Subarray
  3. Maximum Sum Subarray II
  4. Maximum Sum Subarray III
  5. Max Sum Of Three Subarrays
  6. Maximum Sum Subarray of Length K
  7. Maximum Sum Subarray of Length K II
  8. Longest Subarray with Sum at most S
  9. Remove Min Sum

Partial Sums #2

  1. Subarray of Given Sum II
  2. Maximum Sum Submatrix II
  3. Maximum Sum Submatrix III
  4. Remove Min Sum II
  5. Max Sum Of Three Subarrays II
  6. Max Sum Of Three Subarrays III
  7. Max Sum Of Three Subarrays IV
  8. Max Sum Square
  9. Count Submatrices with All Ones

Two Pointers

  1. Palindrome Check
  2. Reverse String
  3. Palindrome Substrings
  4. Palindrome Substrings II
  5. Reverse Words II
  6. Two Sum III
  7. Merge Sorted Arrays
  8. Array Diff
  9. Container with Most Water
  10. Rotate Array By One
  11. Dot Product of Two Sparse Vectors

Hash Table

  1. Hash Map
  2. Collection of Numbers
  3. Ransom Note
  4. Two Sum IV
  5. Hash Set
  6. Distinct Values II
  7. Remove Duplicates From Array III
  8. Find Duplicates
  9. Longest Subarray Without Reapeating II
  10. Longest Subarray with at most K Distinct Integers
  11. Longest Consecutive Sequence II
  12. Group Anagrams II

Binary Search

  1. Binary Search
  2. Lower Bound
  3. Number Of Occurrences
  4. Two Sum II
  5. Find Peak
  6. The Factory
  7. The Factory II
  8. Smallest K V

Recursion

  1. Recursion
  2. Recursive Fibonacci
  3. Power
  4. Decompress File Paths

Divide And Conquer

  1. Binary Search II
  2. Z Pattern
  3. Merge Sort
  4. Quick Sort
  5. Smallest K III

Backtracking

  1. Jump Game
  2. Minimum Path Sum
  3. N Queens
  4. Sudoku
  5. Generate Parentheses
  6. Generate Binary Strings With K Ones
  7. Generate Valley Permutations

Memoization

  1. Memoization
  2. Jump Game II
  3. Jump Game III
  4. Minimum Path Sum II
  5. Minimum Path Sum III

Linked Lists

  1. Linked Lists
  2. Reverse Linked List
  3. Remove Linked List Elements
  4. Remove Nth Node From End Of List
  5. Intersection Of Two Linked Lists
  6. Add Two Numbers
  7. Flatten Multilevel List
  8. Flatten Multilevel List II
  9. Copy List Random Pointer
  10. Copy List Random Pointer II
  11. Merge K Sorted Lists II

Sliding Window

  1. Sliding Window Technique
  2. Longest Subarray with Sum at most S II
  3. Longest Subarray with at most K Distinct Integers II
  4. Longest Subarray Without Reapeating IV
  5. Subarray of Given Sum

Heap

  1. Smallest K II
  2. Last Stone Weight
  3. Kill Monsters
  4. Minimum Number Of Boats

Stack

  1. Stacks
  2. Valid Parentheses
  3. Next Greater Element
  4. Largest Rectangle in Histogram
  5. Reverse Substrings Between Parentheses
  6. Decode String

Binary Trees

  1. Post Order Tree Traversal
  2. Zigzag Tree Traversal
  3. Symmetric Tree
  4. Binary Tree Diameter
  5. Max Sum Bst
  6. Max Sum Bst II
  7. Post Order Tree Traversal II
  8. Zigzag Tree Traversal II
  9. Inorder Tree Traversal
  10. Level Order Traversal
  11. Flatten Binary Tree
  12. Encoded Tree Sums
  13. Convert Sorted Array to Binary Search Tree

Dynamic Programming

  1. Binary Strings Of Given Length
  2. Permutations Of Given Length
  3. Valley Permutations
  4. Binary Strings Without Consecutive Ones
  5. Binary Strings With K Ones
  6. Binary Strings With K Ones On Even Positions
  7. Binary Strings With At Most K Consecutive Ones
  8. Permutations With K Inversions
  9. Digit Product Numbers
  10. Swap Permutations
  11. Best Time To Buy Stock
  12. Best Time To Buy Stock II
  13. Unique Paths
  14. Min Cost Climbing Stairs

Greedy

  1. Non Overlapping Intervals
  2. Meeting Rooms II
  3. Jump Game V
  4. Best Time To Buy Stock III
  5. Find Second Largest II
  6. Maximum Units on a Truck

Graphs

  1. Depth First Search (DFS): Find if Path Exists in Graph
  2. BFS Min Distance to Every Vertex
  3. Number Of Islands
  4. Size Of Islands
  5. 01 Matrix
  6. Shortest Path with Alternating Colors
  7. Dijkstra's Algorithm
  8. Word Ladder
  9. Word Ladder II
  10. Word Ladder III
  11. Word Ladder IV
  12. Jump Game IV

Bit Manipulation

  1. Single Number