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