1. Compute the length of longest valid parentheses
2. Check a binary tree is balanced
3. Make a queue using stacks (Lists in Scala)
4. Find height of binary tree
5. Single-elimination tournament tree
6. Reverse Polish Notation calculator
7. Quick Sort sorting algorithm in pure immutable Scala
8. Find minimum missing positive number in a sequence
9. Least-recently used cache (LRU)
10. Count pairs of a given expected sum
11. Compute a Roman numeral for an Integer, and vice-versa
12. Compute keypad possibilities
13. Matching parentheses algorithm with foldLeft and a state machine
14. Traverse a tree Breadth-First, immutably
15. Read a matrix as a spiral
16. Remove duplicates from a sorted list (state machine)
17. Token Bucket Rate Limiter
18. Leaky Bucket Rate Limiter
19. Merge Sort: stack-safe, tail-recursive, in pure immutable Scala, N-way
20. Longest increasing sub-sequence length
21. Reverse first n elements of a queue
22. Binary search a generic Array
23. Game of Life
24. Merge Sort: in pure immutable Scala
25. Make a queue using Maps
26. Is an Array a permutation?
27. Count number of contiguous countries by colors
28. Add numbers without using addition (plus sign)
29. Tic Tac Toe MinMax solve
30. Run-length encoding (RLE) Encoder
31. Print Alphabet Diamond
32. Find kth largest element in a List
33. Balanced parentheses algorithm with tail-call recursion optimisation
34. Reverse a String's words efficiently
35. Count number of changes (manipulations) needed to make an anagram with an efficient foldLeft
36. Count passing cars
37. Establish execution order from dependencies
38. Counting inversions of a sequence (array) using a Merge Sort
39. Longest common prefix of strings
40. Check if an array is a palindrome
41. Compute missing ranges
42. Check a directed graph has a routing between two nodes (depth-first search)
43. Compute nth row of Pascal's triangle
44. Run-length encoding (RLE) Decoder
45. Check if a number is a palindrome
46. In a range of numbers, count the numbers divisible by a specific integer
47. Compute minimum number of Fibonacci numbers to reach sum
48. Find the index of a substring ('indexOf')
49. Reshape a matrix
50. Compute the steps to transform an anagram only using swaps
51. Compute modulo of an exponent without exponentiation
52. Closest pair of coordinates in a 2D plane
53. Find the contiguous slice with the minimum average
54. Compute maximum sum of subarray (Kadane's algorithm)
55. Pure-functional double linked list
56. Binary search in a rotated sorted array
57. Check if a directed graph has cycles
58. Rotate Array right in pure-functional Scala - using an unusual immutable efficient approach
59. Check a binary tree is a search tree
60. Length of the longest common substring
61. Sliding Window Rate Limiter
62. Tic Tac Toe board check
63. Find an unpaired number in an array
64. Check if a String is a palindrome
65. Count binary gap size of a number using tail recursion
66. Remove duplicates from a sorted list (Sliding)
67. Monitor success rate of a process that may fail
68. Least-recently used cache (MRU)
69. Find sub-array with the maximum sum
70. Find the minimum absolute difference of two partitions
71. Find maximum potential profit from an array of stock price
72. Fibonacci in purely functional immutable Scala
73. Fizz Buzz in purely functional immutable Scala
74. Find triplets that sum to a target ('3Sum')
75. Find combinations adding up to N (non-unique)
76. Find the minimum item in a rotated sorted array
77. Make a binary search tree (Red-Black tree)
78. Remove duplicates from an unsorted List
79. Mars Rover
80. Find combinations adding up to N (unique)
81. Find indices of tuples that sum to a target (Two Sum)
82. Count factors/divisors of an integer
83. Compute single-digit sum of digits
84. Fixed Window Rate Limiter
85. Traverse a tree Depth-First
86. Reverse bits of an integer
87. Find k closest elements to a value in a sorted Array
88. QuickSelect Selection Algorithm (kth smallest item/order statistic)
89. Rotate a matrix by 90 degrees clockwise

1. Class Inside Class
2. Class Inside Def
3. Collect
4. Def Inside Def
5. Drop, Take, dropRight, takeRight
6. foldLeft and foldRight
7. For-comprehension
8. Lazy List
9. Option Type
10. Ordering
11. Partial Function
12. Pattern Matching
13. Range
14. scanLeft and scanRight
15. Sliding / Sliding Window
16. Stack Safety
17. State machine
18. Tail Recursion
19. Type Class
20. View
21. Zip

How the algorithms look

1. A description/goal of the algorithm.
2. An explanation with both Scala and logical parts.
3. A proof or a derivation, where appropriate.
4. Links to Scala concepts used in this specific algorithm, also unit-tested.
5. An implementation in pure-functional immutable Scala, with efficiency in mind (for most algorithms, this is for paid subscribers only).
6. Unit tests, with a button to run them immediately in our in-browser IDE.