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