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. Balanced parentheses algorithm with tail-call recursion optimisation
33. Reverse a String's words efficiently
34. Count number of changes (manipulations) needed to make an anagram with foldLeft and a MultiSet
35. Count passing cars
36. Establish execution order from dependencies
37. Counting inversions of a sequence (array) using a Merge Sort
38. Longest common prefix of strings
39. Check if an array is a palindrome
40. Check a directed graph has a routing between two nodes (depth-first search)
41. Compute nth row of Pascal's triangle
42. Run-length encoding (RLE) Decoder
43. Check if a number is a palindrome
44. In a range of numbers, count the numbers divisible by a specific integer
45. Compute minimum number of Fibonacci numbers to reach sum
46. Find the index of a substring ('indexOf')
47. Reshape a matrix
48. Compute the steps to transform an anagram only using swaps
49. Compute modulo of an exponent without exponentiation
50. Closest pair of coordinates in a 2D plane
51. Find the contiguous slice with the minimum average
52. Compute maximum sum of subarray (Kadane's algorithm)
53. Pure-functional double linked list
54. Binary search in a rotated sorted array
55. Check if a directed graph has cycles
56. Rotate Array right in pure-functional Scala - using an unusual immutable efficient approach
57. Check a binary tree is a search tree
58. Length of the longest common substring
59. Sliding Window Rate Limiter
60. Tic Tac Toe board check
61. Find an unpaired number in an array
62. Check if a String is a palindrome
63. Count binary gap size of a number using tail recursion
64. Remove duplicates from a sorted list (Sliding)
65. Monitor success rate of a process that may fail
66. Least-recently used cache (MRU)
67. Find sub-array with the maximum sum
68. Find the minimum absolute difference of two partitions
69. Find maximum potential profit from an array of stock price
70. Fibonacci in purely functional immutable Scala
71. Fizz Buzz in purely functional immutable Scala
72. Find triplets that sum to a target ('3Sum')
73. Find combinations adding up to N (non-unique)
74. Find the minimum item in a rotated sorted array
75. Make a binary search tree (Red-Black tree)
76. Remove duplicates from an unsorted List
77. Mars Rover
78. Find combinations adding up to N (unique)
79. Find indices of tuples that sum to a target (Two Sum)
80. Count factors/divisors of an integer
81. Compute single-digit sum of digits
82. Fixed Window Rate Limiter
83. Traverse a tree Depth-First
84. Reverse bits of an integer
85. Find k closest elements to a value in a sorted Array
86. QuickSelect Selection Algorithm (kth smallest item/order statistic)
87. 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.