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Part 1 · Learn to code. Coding fundamentals, from your first line of code to solving problems on your own: loops, lists, strings, functions and the basics of classes.

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Part 1 · Chapter 1

Welcome To Kotlin

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01 Printing to the Console Free
02 Intro to Strings Free
Comments PRO
04 Intro to Variables Free
05 Syntax Errors Free
Name Errors PRO
07 Strings Vs Numbers Free
String Concatenation PRO

Part 1 · Chapter 2

Variables

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The Power Of Variables PRO
10 Data Types Free
Variable Naming Rules PRO
12 Variable Reassignment Free
Constants PRO
val vs var PRO
Type Conversion PRO

Part 1 · Chapter 3

Calculations With Numbers

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16 Arithmetic Operators Free
Area Of Rectangle PRO
Mathematical Expressions PRO
Find Discount PRO
20 Operator Precedence Free
21 Price After Discount Free
Convert Hours And Mins Into Seconds PRO
Floor Division PRO
Free Coffee Cups PRO
Modulo Operator PRO
Augmented Assignment Operators PRO
Increment And Decrement Operators PRO

Part 1 · Chapter 4

Functions I

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Functions PRO
Defining Functions: Buggy Code PRO
Defining Functions: Quiz PRO
Calling Functions: Buggy Code PRO
Calling Functions: Quiz PRO
Execution Flow: Quiz PRO
Why Functions PRO
Exercise: Create a Function PRO
Function Parameters & Arguments PRO
Parameters & Arguments: Buggy Code I PRO
Parameters & Arguments: Buggy Code II PRO
Why Parameters & Arguments PRO
Exercise: Create Function With Parameters PRO
Returning From Functions PRO
Returning From Functions: Quiz I PRO
Returning From Functions: Buggy Code I PRO
Returning From Functions: Quiz II PRO
Returning From Functions: Buggy Code II PRO
Returning From Functions: Quiz III PRO
Returning From Functions: Buggy Code III PRO
Returning From Functions: Quiz IV PRO
Returning From Functions: Buggy Code IV PRO

Part 1 · Chapter 5

Calculations - Challenges

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Your First Challenge PRO
Buy Candy PRO
Hand Shakes PRO
Last Two Digit Sum PRO
Employee of the Year 1: One Employee PRO

Part 1 · Chapter 6

Strings I

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Escaping Characters PRO
String Concatenation II PRO
Accessing Characters PRO
String Slicing PRO
String Immutability PRO
String Length PRO
Exceeding String Bounds PRO
String Find PRO
String Replace PRO
String Split PRO
String Strip / Trim PRO
Uppercase and Lowercase PRO
Top K Brands 1: One Brand PRO

Part 1 · Chapter 7

Strings - Challenges

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Concat First and Last Char PRO
Amazing to Not Amazing PRO
Creating IDs PRO
Stuttering Function PRO
Repeat Last Char PRO
Burrrrp PRO
File Extension PRO
Initials PRO

Part 1 · Chapter 8

Conditional Statements

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76 If Statements Free
77 If-Else Statements Free
78 Comparison Operators Free
79 Is Number Negative Free
80 Student Passed Free
Return Negative PRO
Redundant Else Statement PRO
Parity of Number PRO
And Operator PRO
Reducing If Else To Boolean Expression PRO
Allow to Contest PRO
Or Operator PRO
Not Operator PRO
Combining Logical Operators PRO
90 Chaining Conditional Statements Free
Null Safety PRO
when Expressions PRO

Part 1 · Chapter 9

Conditional Statements - Challenges

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Recreating ABS PRO
Recreating Absolute Difference PRO
Leap Year PRO
Interval Intersection PRO
Second Largest Value PRO
Employee of the Year 2: Two Employees PRO
Employee of the Year 3: Three Employees PRO
Top K Brands 2: Better of Two PRO

Part 1 · Chapter 10

Arrays I

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Static Arrays PRO
ArrayLists PRO
Accessing Static Array Elements PRO
Accessing ArrayLists Elements PRO
Updating Static Array Elements PRO
Updating ArrayLists Elements PRO
Static Array Length PRO
ArrayLists Size PRO
Exceeding Static Array Bounds PRO
Exceeding ArrayLists bounds PRO
Append Item to Array PRO
Remove Last Item from Array PRO
Insert and Remove at a Position PRO
Checking an Array for a Value PRO
Slicing an Array PRO

Part 1 · Chapter 11

Arrays - Challenges

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Center of the Grid PRO
Swap First and Last PRO
Middle Element PRO
Undo the Last Add PRO
First n Items PRO
Drop the Ends PRO
Move to the Front PRO
Add If Missing PRO

Part 1 · Chapter 12

Dictionaries

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Dictionaries (HashMap) PRO
Accessing Dictionary Values PRO
Updating Dictionary Values PRO
Dictionary Length PRO
Checking for a Key PRO
Reading a Key That May Be Missing PRO
Deleting a Key PRO

Part 1 · Chapter 13

Dictionaries - Challenges

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Phone Book Lookup PRO
Restock PRO
Safe Lookup PRO
Add a Contact PRO
Sell an Item PRO
Count a Vote PRO

Part 1 · Chapter 14

For Loops - Looping Over Arrays

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Why For Loops? PRO
For Loop - Looping over Arrays PRO
Looping over Arrays: Buggy Code I PRO
Looping over Arrays: Buggy Code II PRO
For Loop - Execution Flow PRO
For Loop - Execution Flow: Buggy Code I PRO
For Loop - Execution Flow: Buggy Code II PRO
For Loop - Execution Flow: Buggy Code III PRO
Infinite For Loops PRO

Part 1 · Chapter 15

While Loops & For Loops

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Why Loops? PRO
The While Loop PRO
While Loop: Initialization & Condition PRO
While Loop: Iteration PRO
Infinite While Loops PRO
The Classic For Loop PRO
For Loop: Initialization & Condition PRO
For Loop: Iteration PRO
Looping in Reverse PRO
Infinite For Loops II PRO
Print Multiples PRO

Part 1 · Chapter 16

Loops & Conditionals

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Positive Numbers From Array PRO
Even Numbers From Array: Buggy Code PRO
Print Even-Odd Numbers PRO
Print Odd Numbers: Buggy Code PRO
Print Positive-Negative Numbers: Buggy Code PRO
Array Contains Number PRO
Positive Number in Array: Buggy Code PRO
Hurdle Jump PRO
Prime Number PRO
Collatz Conjecture PRO

Part 1 · Chapter 17

Loops & Global Variables

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Sum of Numbers PRO
Sum of Numbers: Buggy Code PRO
Sum of Squares PRO
Factorial PRO
Array Sum: Buggy Code PRO
Student Grades PRO
Number Of Occurences In Array PRO
H4ck3r Sp34k PRO
Maximum Number In Array PRO
Check If Odd > Even PRO

Part 1 · Chapter 18

Looping Over Array Indices

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Looping Through Arrays With Indices PRO
Number Index In Array PRO
Lower Bound Index PRO
Manipulating Array Elements inside Loop PRO
Get Full Names I PRO
Reverse Array PRO
Break In For Loops PRO
Continue In For Loops PRO
Employee of the Year 4: Any Number of Employees PRO
Employee of the Year 5: Top 2 PRO
Top K Brands 3: Best in a List PRO

Part 1 · Chapter 19

Kotlin Essentials

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val vs var PRO
Null Safety PRO
when Expressions PRO
Data Classes PRO

Part 1 · Chapter 20

Projects: Everyday Programs

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Shopping Cart 1: Single Item Total PRO
Shopping Cart 2: Tax and Discount PRO
Shopping Cart 3: Free Shipping PRO
Shopping Cart 4: Cart Sum PRO
Shopping Cart 5: Coupon Codes PRO
Shopping Cart 6: Group by Category PRO
Password Strength 1: Length Check PRO
Password Strength 2: Has a Digit PRO
Password Strength 3: Character Classes PRO
Password Strength 4: Count Character Classes PRO
Password Strength 5: Common Passwords PRO
Password Strength 6: Score from 0 to 100 PRO
Report Card 1: One Subject Average PRO
Report Card 2: Weighted Score PRO
Report Card 3: Letter Grade PRO
Report Card 4: Multiple Students PRO
Report Card 5: Grades by Subject PRO
Report Card 6: Class Statistics PRO

Part 1 · Chapter 21

Loops - Challenges

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Print Powers PRO
Fizz BuzzO(n) PRO
Longest Common Prefix Of Two Strings PRO
Get Full Names II PRO
Find Largest Number PRO
Fibonacci PRO
Tribonacci PRO
215 Max Val And Number Of OccurrencesO(n) Free

Part 1 · Chapter 22

While Loops - Challenges

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Digit Sum PRO
Number of Digits PRO
Reverse a Number PRO
Halving Steps PRO
Sum Until Limit PRO

Part 1 · Chapter 23

Loops With Strings, Arrays & Dictionaries

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Count the Vowels PRO
Capitalize Each Word PRO
Long Words Only PRO
Word Lengths PRO
Longest Word PRO
Looping Over a Dictionary PRO
Top Scorer PRO
Letter Count PRO
Word Frequency PRO
Most Frequent Element PRO
Two Arrays to a Dictionary PRO
Invert a Dictionary PRO
Group Words by First Letter PRO

Part 1 · Chapter 24

Sets

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Sets PRO
Unique Values PRO
First Repeat PRO
Common Values PRO
Removing from a Set and Looping Over It PRO
Guest Check-In PRO
Remove Repeats PRO
Only in the First PRO
Every Letter PRO

Part 1 · Chapter 25

Helper Functions

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Max of Three with a Helper PRO
Letter Grades PRO
Count the Primes PRO
Clamp the Values PRO
Palindrome Words PRO
Biggest Digit Sum PRO
Perfect Numbers PRO

Part 1 · Chapter 26

Nested Loops

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Nested Loops PRO
Print Triangle Of StarsO(n²) PRO
Print RhombusO(n²) PRO
Print XO(n²) PRO
Employee of the Year 6: Top K PRO
Top K Brands 4: Top K PRO
Top K Brands 5: Filter by Product PRO
Top K Brands 6: Tie-Break by Reviews PRO

Part 1 · Chapter 27

Nested Loops Over Grids

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Grid Sum PRO
Row Sums PRO
Column Sums PRO
Count in a Grid PRO
Find in a Grid PRO
Diagonal Sum PRO

Part 1 · Chapter 28

Pairs, Sorting & Built-ins

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Pairs PRO
Smallest and Largest PRO
Oldest Pet PRO
Custom Sorting PRO
Shortest Words First PRO
Leaderboard PRO
Map and Filter PRO
Shout! PRO
Squares of the Positives PRO
Min, Max and Sum PRO
Total Without Extremes PRO
withIndex and zip PRO
Price List PRO

Part 1 · Chapter 29

Strings

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277 Intro To Strings You're here
Strings PRO
Reverse String IIO(n²) PRO
Reverse String IIIO(n) PRO
Palindrome Check IIO(n) PRO
Reverse WordsO(n) PRO

Part 1 · Chapter 30

Functions & Classes

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Classes PRO
284 Magical NumberO(log n) Free
Defining a Class PRO
Creating Objects PRO
Constructors PRO
Class Fields PRO
Class Methods PRO
RectangleO(1) PRO
Score KeeperO(1) PRO
Water BottleO(1) PRO

Part 2 · Chapter 1

Free Coding Interview Questions

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96 Minimum Value of Three Free
215 Max Val And Number Of OccurrencesO(n) Free
317 Maximum Sum SubarrayO(n²) Free
565 Number Of IslandsO(n * m) Free
518 Binary Strings With At Most K Consecutive OnesO(n * k) Free

Part 2 · Chapter 2

Big O Notation

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Linear Time Complexity PRO
Constant Time Complexity PRO
Built-in Functions - Time Complexity PRO
Array & String Methods - Time Complexity PRO
Slicing & Concatenation - Time Complexity PRO
Quadratic Time Complexity PRO
Space Complexity PRO

Part 2 · Chapter 3

Brute Force

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Pair CountO(n²) PRO
Two SumO(n²) PRO
Selection SortO(n²) PRO
Remove Duplicates From ArrayO(n²) PRO
Longest Subarray Without ReapeatingO(n³) PRO
Minimum Absolute DifferenceO(n²) PRO
Find The DifferenceO(n²) PRO

Part 2 · Chapter 4

Sorting

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Sorting PRO
Smallest KO(n log n) PRO
Distinct ValuesO(n log n) PRO
Remove Duplicates From Array IIO(n log n) PRO
Longest Consecutive SequenceO(n log n) PRO
Valid AnagramO(n log n) PRO
Group AnagramsO(n * log n * L * log L) PRO
Find Second LargestO(n log n) PRO
Minimum Absolute Difference IIO(n log n) PRO

Part 2 · Chapter 5

Partial Sums #1

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Partial Sums PRO
317 Maximum Sum SubarrayO(n²) Free
Maximum Sum Subarray IIO(n) PRO
Maximum Sum Subarray IIIO(n) PRO
Max Sum Of Three SubarraysO(n⁶) PRO
Maximum Sum Subarray of Length KO(n * k) PRO
Maximum Sum Subarray of Length K IIO(n) PRO
Longest Subarray with Sum at most SO(n²) PRO
Product of Array Except SelfO(n) PRO
Remove Min SumO(n) PRO

Part 2 · Chapter 6

Partial Sums #2

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Subarray of Given Sum IIO(n) PRO
Maximum Sum Submatrix IIO(n² * m²) PRO
Maximum Sum Submatrix IIIO(n² * m) PRO
Remove Min Sum IIO(n) PRO
Max Sum Of Three Subarrays IIO(n³) PRO
Max Sum Of Three Subarrays IIIO(n²) PRO
Max Sum Of Three Subarrays IVO(n) PRO
Max Sum SquareO(n² * m) PRO
Count Submatrices with All OnesO(n^3) PRO

Part 2 · Chapter 7

Matrix

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Transpose MatrixO(m · n) PRO
Toeplitz MatrixO(m · n) PRO
Rotate ImageO(n^2) PRO
Spiral MatrixO(m*n) PRO
Spiral Matrix IIO(n²) PRO
Set Matrix ZeroesO(m*n) PRO
Game of LifeO(m · n) PRO
Search a 2D Matrix IIO(m + n) PRO

Part 2 · Chapter 8

Two Pointers

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Palindrome CheckO(n) PRO
Reverse StringO(n) PRO
Palindrome SubstringsO(n³) PRO
Palindrome Substrings IIO(n²) PRO
Reverse Words IIO(n) PRO
Two Sum IIIO(n log n) + O(n) PRO
Two Sum II - Input Array Is SortedO(n) PRO
Merge Sorted ArraysO(n + m) PRO
Array DiffO(n log n) PRO
Container with Most WaterO(n) PRO
3SumO(n²) PRO
Rotate Array By OneO(n) PRO
Rotate ArrayO(n) PRO
Dot Product of Two Sparse VectorsO(n) PRO

Part 2 · Chapter 9

Hash Table

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Hash Map PRO
Collection of NumbersO(1) PRO
Ransom NoteO(n + m) PRO
Two Sum IVO(n) PRO
Hash Set PRO
Distinct Values IIO(n) PRO
Remove Duplicates From Array IIIO(n) PRO
Find DuplicatesO(n) PRO
Valid SudokuO(81) PRO
Longest Subarray Without Reapeating IIO(n²) PRO
Longest Subarray with at most K Distinct IntegersO(n²) PRO
Longest Consecutive Sequence IIO(n) PRO
Group Anagrams IIO(n * L * log L) PRO
Encode and Decode StringsO(n) PRO
Find The Difference IIO(n) PRO
Intersection of Two ArraysO(n + m) PRO
Top K Frequent ElementsO(n) PRO
LRU Cache PRO
Design Underground SystemO(1) PRO

Part 2 · Chapter 10

Binary Search

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Binary SearchO(log n) PRO
Lower BoundO(log n) PRO
Number Of OccurrencesO(log n) PRO
Search a 2D MatrixO(log(m · n)) PRO
Two Sum IIO(n log n) PRO
Find PeakO(log n) PRO
The FactoryO(n * k * T) PRO
The Factory IIO(n * log(k * T)) PRO
Koko Eating BananasO(n log m) PRO
Uber RidesO(n log T) PRO
Smallest K VO(n log n) PRO
Search in Rotated Sorted ArrayO(log n) PRO
Find Minimum in Rotated Sorted ArrayO(log n) PRO
Median of Two Sorted ArraysO(log(min(m, n))) PRO
Time Based Key-Value StoreO(log n) PRO

Part 2 · Chapter 11

Recursion

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RecursionO(n) PRO
Sum of a List, RecursivelyO(n) PRO
Reverse a String, RecursivelyO(n) PRO
Palindrome, RecursivelyO(n) PRO
Recursive FibonacciO(2ⁿ) PRO
PowerO(log n) PRO
Tower of HanoiO(2ⁿ) PRO
Flatten a Nested ListO(n) PRO
590 Decompress File Paths Free

Part 2 · Chapter 12

Divide And Conquer

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Binary Search IIRecursive PRO
Z PatternO(n²) PRO
Merge SortO(n log n) PRO
Majority ElementO(n log n) PRO
Count InversionsO(n log n) PRO
Quick SortO(n log n) PRO
Smallest K IIIO(n) PRO

Part 2 · Chapter 13

Backtracking

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Jump GameO(2ⁿ) PRO
Minimum Path SumO(2ⁿ) PRO
N QueensO(n!) PRO
SudokuO(n^n²) PRO
Word SearchO(R · C · 4^L) PRO
Letter Combinations of a Phone NumberO(n · 4ⁿ) PRO
Generate ParenthesesO(2^n) PRO
Generate Binary Strings With K OnesO(2^n) PRO
SubsetsO(n · 2ⁿ) PRO
Combination SumO(n^t/m) PRO
Subsets IIO(n · 2ⁿ) PRO
Combination Sum IIO(n · 2ⁿ) PRO
Palindrome PartitioningO(n · 2ⁿ) PRO
PermutationsO(n · n!) PRO
Generate Valley PermutationsO(n!) PRO

Part 2 · Chapter 14

Memoization

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MemoizationO(n) PRO
Jump Game IIO(n²) - Memoization PRO
Jump Game IIIO(n²) - DP PRO
Minimum Path Sum IIO(n²) - Memoization PRO
Minimum Path Sum IIIO(n²) - DP PRO
Target SumO(n · total) PRO
Interleaving StringO(m · n) PRO
Longest Increasing Path in a MatrixO(m · n) PRO

Part 2 · Chapter 15

Linked Lists

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Linked Lists PRO
Reverse Linked ListO(n) PRO
Remove Linked List ElementsO(n) PRO
Remove Nth Node From End Of ListO(n) PRO
Remove Duplicates From Sorted Linked ListO(n) PRO
Linked List CycleO(n) PRO
Find the Duplicate NumberO(n) PRO
Palindrome Linked ListO(n) PRO
Intersection Of Two Linked ListsO(n) PRO
Add Two NumbersO(n) PRO
Flatten Multilevel ListO(n) Recursive PRO
Flatten Multilevel List IIO(n) PRO
Copy List Random PointerO(n) PRO
Copy List Random Pointer IIO(n) PRO
Merge Two Sorted ListsO(n + m) PRO
Reorder ListO(n) PRO
Reverse Nodes in k-GroupO(n) PRO
Merge K Sorted ListsO(n × k) PRO
Merge K Sorted Lists IIO(N log k) PRO

Part 2 · Chapter 16

Sliding Window

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Sliding Window Technique PRO
Longest Subarray with Sum at most S IIO(n) PRO
Longest Subarray with at most K Distinct Integers IIO(n) PRO
Longest Subarray Without Reapeating IVO(n) PRO
Subarray of Given SumO(n) PRO
Longest Repeating Character ReplacementO(n) PRO
Permutation in StringO(n) PRO
Minimum Window SubstringO(n) PRO
Sliding Window MaximumO(n) PRO

Part 2 · Chapter 17

Heap

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Smallest K IIO(n log k) PRO
K Closest Points to OriginO(n log k) PRO
Kth Largest Element in a StreamO(log k) PRO
Last Stone WeightO(n log n) PRO
Task SchedulerO(T log 26) PRO
Kill MonstersO(n log n) PRO
Minimum Number Of BoatsO(n log n) PRO
Find Median from Data StreamO(log n) PRO
Design TwitterO(F log F) PRO

Part 2 · Chapter 18

Stack

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Stacks PRO
Valid ParenthesesO(n) PRO
Evaluate Reverse Polish NotationO(n) PRO
Min StackO(1) PRO
Implement Queue using StacksO(1) amortized PRO
Design Browser HistoryO(1) PRO
Next Greater ElementO(n) PRO
Car FleetO(n log n) PRO
Remove K DigitsO(n) PRO
Create Maximum NumberO(n) PRO
Largest Rectangle in HistogramO(n) PRO
Reverse Substrings Between ParenthesesO(n^2) PRO
Decode StringO(n) PRO

Part 2 · Chapter 19

Binary Trees

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Post Order Tree TraversalO(n) PRO
Zigzag Tree TraversalO(n) PRO
Symmetric TreeO(n) PRO
Lca In BstO(n) PRO
Binary Tree DiameterO(n) PRO
Binary Tree Maximum Path SumO(n) PRO
Kth Smallest Element In BstO(n) PRO
Validate Binary Search TreeO(n) PRO
Count Good Nodes in Binary TreeO(n) PRO
Max Sum BstO(n²) PRO
Max Sum Bst IIO(n) PRO
Post Order Tree Traversal IIO(n) PRO
Maximum Depth of Binary TreeO(n) PRO
Balanced Binary TreeO(n) PRO
Invert Binary TreeO(n) PRO
Same TreeO(n) PRO
Subtree of Another TreeO(n · m) PRO
Recover Tree From PreorderO(n) PRO
Construct Binary Tree from Preorder and Inorder TraversalO(n) PRO
Serialize and Deserialize Binary TreeO(n) PRO
Zigzag Tree Traversal IIO(n) PRO
Inorder Tree TraversalO(n) PRO
Level Order TraversalO(n) PRO
Binary Tree Right Side ViewO(n) PRO
Flatten Binary TreeO(n) PRO
Encoded Tree SumsO(n) PRO
Convert Sorted Array to Binary Search TreeDivide and Conquer, O(n) PRO

Part 2 · Chapter 20

Tries

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Longest Common PrefixO(S) PRO
Implement Trie (Prefix Tree)O(L) PRO
Replace WordsO(S) PRO
Design Add and Search Words Data StructureO(L) PRO
Search Suggestions SystemO(S log n + m) PRO
Word Search IIO(R · C · 4^L) PRO

Part 2 · Chapter 21

Dynamic Programming

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Binary Strings Of Given LengthO(n) PRO
Permutations Of Given LengthO(n) PRO
Valley PermutationsO(n) PRO
Binary Strings Without Consecutive OnesO(n) PRO
Binary Strings With K OnesO(n * k) PRO
Binary Strings With K Ones On Even PositionsO(n * k) PRO
518 Binary Strings With At Most K Consecutive OnesO(n * k) Free
Permutations With K InversionsO(n * k) PRO
Digit Product NumbersO(n * p * digits) PRO
Swap PermutationsO(n) PRO
Best Time To Buy StockO(n³) PRO
Best Time To Buy Stock IIO(n²) PRO
Trapping Rain WaterO(n) PRO
Unique PathsO(n * m) PRO
Climbing StairsO(n) PRO
Min Cost Climbing StairsO(n) PRO
House RobberO(n) PRO
House Robber IIO(n) PRO
Decode WaysO(n) PRO
Maximum Product SubarrayO(n) PRO
Best Time to Buy and Sell Stock with CooldownO(n) PRO
Coin ChangeO(n · amount) PRO
Coin Change IIO(n · amount) PRO
Partition Equal Subset SumO(n · sum) PRO
Word BreakO(n · m · k) PRO
Longest Common SubsequenceO(n · m) PRO
Edit DistanceO(m · n) PRO
Distinct SubsequencesO(m · n) PRO
Regular Expression MatchingO(m · n) PRO
Longest Increasing SubsequenceO(n²) PRO
Longest Palindromic SubstringO(n²) PRO
Burst BalloonsO(n³) PRO

Part 2 · Chapter 22

Greedy

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Lemonade ChangeO(n) PRO
Jump Game: Reach the EndO(n) PRO
Jump Game VO(n) PRO
Best Time To Buy Stock IIIO(n) PRO
Find Second Largest IIO(n) PRO
Maximum Units on a TruckGreedy, O(n log n) PRO
Merge Triplets to Form Target TripletO(n) PRO
Gas StationO(n) PRO
Partition LabelsO(n) PRO
Hand of StraightsO(n log n) PRO
Valid Parenthesis StringO(n) PRO

Part 2 · Chapter 23

Intervals

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Can Attend All MeetingsO(n log n) PRO
Merge IntervalsO(n log n) PRO
Insert IntervalO(n) PRO
Interval List IntersectionsO(m + n) PRO
Non Overlapping IntervalsO(n log n) PRO
Fewest Arrows to Burst BalloonsO(n log n) PRO
Meeting Rooms IIO(n log n) PRO
Minimum Interval to Include Each QueryO(n log n + q log q) PRO

Part 2 · Chapter 24

Graphs

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Depth First Search (DFS): Find if Path Exists in GraphO(V+E) PRO
BFS Min Distance to Every VertexBFS, O(V+E) PRO
565 Number Of IslandsO(n * m) Free
Size Of IslandsO(n * m) PRO
Number of Connected Components in an Undirected GraphO((n + e)·α(n)) PRO
Graph Valid TreeO((n + e)·α(n)) PRO
Redundant ConnectionO(n · α(n)) PRO
01 MatrixO(n * m) PRO
Rotting OrangesO(m · n) PRO
Walls and GatesO(m · n) PRO
Shortest Path with Alternating ColorsBFS, O(V+E) PRO
Course ScheduleO(V + E) PRO
Course Schedule IIO((V + E) log V) PRO
Alien DictionaryO(C) PRO
Reconstruct ItineraryO(E log E) PRO
Clone GraphO(V + E) PRO
Pacific Atlantic Water FlowO(m · n) PRO
Surrounded RegionsO(m · n) PRO
Dijkstra's AlgorithmPriority Queue, O((V+E) log V) PRO
Cheapest Flights Within K StopsO(k · E) PRO
Swim in Rising WaterO(n² log n) PRO
Min Cost to Connect All PointsO(n² log n) PRO
Word LadderO(N² * L) PRO
Word Ladder IIO(N * L² * Σ) PRO
Word Ladder IIIO(N * L²) PRO
Word Ladder IVO(N * L²) PRO
Jump Game IVO(n) PRO

Part 2 · Chapter 25

Bit Manipulation

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Find The Difference IIIO(n) / O(1) PRO
Single NumberO(n log n) PRO
Single Number IIO(n) PRO
Number of 1 BitsO(k) PRO
Counting BitsO(n) PRO
Missing NumberO(n) PRO
Reverse BitsO(1) PRO
Sum of Two IntegersO(1) PRO

Part 2 · Chapter 26

Math

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Plus OneO(n) PRO
Roman to IntegerO(n) PRO
Reverse IntegerO(log |x|) PRO
Happy NumberO(log n) PRO
Multiply StringsO(m · n) PRO
Detect SquaresO(n) PRO

Part 3 · Chapter 1

Projects: Command-Line Tools

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cat 1: Print Lines PRO
cat 2: Number Lines (-n) PRO
cat 3: Show Special Characters (-A) PRO
cat 4: Join Several Files PRO
head 1: First 10 Lines PRO
head 2: First n Lines PRO
head 3: When n Is Too Big PRO
head 4: Negative n PRO
wc 1: Count Lines PRO
wc 2: Count Words (-w) PRO
wc 3: Count Characters (-c) PRO
wc 4: Choose What to Count PRO
grep 1: Lines That Match PRO
grep 2: Ignore Case (-i) PRO
grep 3: Lines That Don't Match (-v) PRO
grep 4: Whole Words (-w) PRO
uniq 1: Drop Repeated Lines PRO
uniq 2: Count Repeats (-c) PRO
uniq 3: Only Repeated Lines (-d) PRO
uniq 4: Only Unique Lines (-u) PRO
sort 1: Alphabetical Order PRO
sort 2: Reverse Order (-r) PRO
sort 3: Numeric Order (-n) PRO
sort 4: Sort by Column (-k) PRO
tr 1: Swap One Character PRO
tr 2: Translate a Set PRO
tr 3: Delete Characters (-d) PRO
tr 4: Squeeze Repeats (-s) PRO

Part 3 · Chapter 2

Object-Oriented Programming

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Static Members: Count the Dogs PRO
Access Modifiers PRO
Secret PIN PRO
Getters and Setters PRO
Inheritance PRO
Animal Sounds PRO
Method Overriding PRO
Custom Greeting PRO
Composition: Has-a PRO
Calling the Parent Method PRO
Polymorphism PRO
Pay Everyone PRO
Abstract Classes PRO
Shape Area PRO
Interfaces PRO
Printable Things PRO
Printing Objects PRO
Comparing Objects PRO
Errors and Exceptions PRO
OOP Capstone PRO
Data Classes PRO

Part 3 · Chapter 3

Object-Oriented Programming - Challenges

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Ticket NumbersO(1) PRO
ThermostatO(1) PRO
Inventory ItemO(1) PRO
Product PriceO(1) PRO
Employees and ManagersO(1) PRO
Shipping OptionsO(1) PRO
Car and EngineO(1) PRO
ShapesO(n) PRO
Payment FeesO(1) PRO
NotifiersO(n) PRO
Book LabelO(1) PRO
Sort Your Own ObjectsO(n log n) PRO
Unique PointsO(n) PRO
Ticket BookingO(1) PRO
PlaylistO(n) PRO

Part 3 · Chapter 4

Projects: Object-Oriented

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Bank Account (Encapsulation) PRO
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Shopping Cart Classes PRO
Vending Machine PRO
Tic-Tac-Toe Game Class PRO

Part 3 · Chapter 5

Object-Oriented Design - Interviews

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Parking LotO(log n) PRO
Library CheckoutO(1) PRO
Deck of CardsO(n) PRO
ElevatorO(1) PRO
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Part 4 · Chapter 1

Foundations

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01 What Happens When You Open a LinkDNS, request, server, response Free
02 APIsMethods, paths, status codes Free
03 Latency NumbersMemory, disk, network Free
04 Back-of-the-Envelope NumbersRequests a second, storage, peaks Free
05 Scaling Up vs Scaling OutStateless servers, sessions Free
06 The Interview FrameworkThe 8 steps, on a like button Free

Part 4 · Chapter 2

Building Blocks

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Load BalancingRound robin, least connections PRO
CachingCache-aside, hit rate, TTL PRO
Eviction: LRU and LFU PRO
When Caches Go WrongStale data, stampedes PRO
CDNs PRO
SQL or NoSQL PRO
Indexes PRO
ReplicationLeaders, followers, lag PRO
ShardingKeys, ranges, hot keys PRO
Consistent HashingThe ring, virtual nodes PRO
CAP and ConsistencyQuorums, PACELC PRO
Message QueuesRetries, idempotency PRO
Pub/Sub and Streams PRO
Rate LimitingToken bucket, windows PRO
Unique IDsUUID, Snowflake PRO
Files and Object Storage PRO
Real-Time UpdatesPolling, WebSockets PRO
Concurrency and LocksThe last seat PRO

Part 4 · Chapter 3

Engineering Craft

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API Versioning PRO
PaginationOffset vs cursor PRO
Authentication and API KeysIncluding key rotation PRO
Idempotency Keys PRO
Webhooks PRO
Data Modeling PRO
Tables From Query Patterns PRO
Transactions and Isolation PRO
Timeouts, Retries, Circuit Breakers PRO
Monitoring and SLOs PRO
Safe DeploymentsCanary, feature flags PRO
Multi-Region and Disaster Recovery PRO
Networking BasicsTCP, UDP, HTTP, TLS PRO
Consensus and Leader Election PRO

Part 4 · Chapter 4

Design: Read-Heavy & Counting

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39 Design a URL ShortenerCaching, unique codes, 301 vs 302 Free
Rate Limiter PRO
Key-Value Store PRO
Distributed CounterViews, likes PRO
Top K / Leaderboard PRO

Part 4 · Chapter 5

Design: Social & Feeds

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Twitter at ScaleFan-out, celebrities PRO
Instagram PRO
Reddit PRO
Live Comments PRO

Part 4 · Chapter 6

Design: Messaging & Real Time

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Chat (WhatsApp) PRO
Notification System PRO

Part 4 · Chapter 7

Design: Media & Files

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YouTube / Netflix PRO
Spotify PRO
Dropbox / Google Drive PRO

Part 4 · Chapter 8

Design: Search & Data

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Autocomplete PRO
Web Crawler PRO
Search Engine PRO
Ad Click Aggregator PRO
Metrics and Alerting PRO

Part 4 · Chapter 9

Design: Location

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Nearby Places (Yelp) PRO
Uber PRO
Google Maps PRO
Food Delivery (DoorDash) PRO

Part 4 · Chapter 10

Design: Booking & Money

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Ticketmaster PRO
Hotel Booking PRO
Flash Sale PRO
Payment System PRO
Digital Wallet PRO
Stock Exchange PRO
Online Auction (eBay) PRO

Part 4 · Chapter 11

Design: Infrastructure

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Message Queue (Kafka) PRO
Job Scheduler PRO
Google Docs PRO

Part 4 · Chapter 12

Design: Developer Tools

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Design LeetCodeAn online judge PRO
CI/CD (GitHub Actions) PRO

Part 4 · Chapter 13

Design: AI Systems

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ChatGPTServing an LLM PRO
Support Bot With RAG PRO
AI Code Assistant PRO

Part 4 · Chapter 14

Practice Problems

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Part 5 · Chapter 1

Season 1: Forms

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01 Ruth · Library sign-up pageMaple Street Library Free
Marcus · Bake sale order formRiverside Elementary PTA PRO
Mr. Okafor · Grade calculatorLincoln Middle School PRO
Sam · Soccer club sign-upNorthside Youth Soccer PRO
Priya · Yoga class bookingSunrise Yoga PRO
Tom · Café bill splitterThe Corner Café PRO
Lena · Dog walking pricesHappy Paws Dog Walking PRO
Carmen · Food truck pre-ordersCarmen’s Tacos PRO

Part 5 · Chapter 2

Season 2: Lists and saved data

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Mr. Brennan · School lunch weekOak Hill Elementary PRO
Joan · Donor list cleanupNorthside Food Bank PRO
Dmitri · Painting quotesFresh Coat Painting PRO
Ms. Kowalski · Chess tournament pairingsRiverside Chess Club PRO
Grace · Bike rental deskSpoke & Pedal Bike Rental PRO

Part 5 · Chapter 3

Your own app

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14 Your own appYour idea. This time you’re the client.

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LESSON 277 OF 677 · STRINGS

Introduction to Strings

Reading

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