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