Scala algorithm: Reverse a String's words efficiently

Published

Algorithm goal

Efficiently, reverse a String's words. For example, 'Hello new world' becomes 'world new Hello'.

Test cases in Scala

assert(reverseWords("").toList == List(""))
assert(reverseWords("A").toList == List("A"))
assert(reverseWords("AA").toList == List("AA"))
assert(reverseWords("AA BB").toList == List("BB", "AA"))
assert(reverseWords("AA  BB").toList == List("BB", "", "AA"))
assert(reverseString("cannot") == "cannot")
assert(reverseString("") == "")
assert(reverseString(" a b ") == " b a ")
assert(reverseString("AA BB") == "BB AA")
assert(reverseString("cannot do") == "do cannot")
assert(
  reverseString("cannot do without Scala") == "Scala without do cannot"
)

Algorithm in Scala

34 lines of Scala (compatible versions 2.13 & 3.0), showing how concise Scala can be!

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Explanation

While this may look a long-winded solution, it is very explicit and also streaming-based.

An efficiency portion of this algorithm comes from the fact that rather than appending every character that we come across to an existing string, and thus ending up with allocations of a new String per character, we can extract out the positions of the spaces, and come up with the shape of our target String by describing it in terms of the edges and where the spaces are. (this is © from www.scala-algorithms.com)

Full explanation is available to subscribers Scala algorithms logo, maze part, which looks quirky

Scala concepts & Hints

  1. Collect

    'collect' allows you to use Pattern Matching, to filter and map items.

    assert("Hello World".collect {
  case character if Character.isUpperCase(character) => character.toLower
} == "hw")

  2. Pattern Matching

    Pattern matching in Scala lets you quickly identify what you are looking for in a data, and also extract it.

    assert("Hello World".collect {
  case character if Character.isUpperCase(character) => character.toLower
} == "hw")

  3. Sliding / Sliding Window

    Get fixed-length sliding sub-sequences (sliding windows) from another sequence

  4. State machine

    A state machine is the use of `sealed trait` to represent all the possible states (and transitions) of a 'machine' in a hierarchical form.

  5. View

    The .view syntax creates a structure that mirrors another structure, until "forced" by an eager operation like .toList, .foreach, .forall, .count.

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Fully unit-tested, with explanations and relevant concepts; new algorithms published about once a week.

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  6. Find height of binary tree
  7. Single-elimination tournament tree
  8. Reverse Polish Notation calculator
  9. Quick Sort sorting algorithm in pure immutable Scala
  10. Check word in grid (depth-first search)
  11. Maximum wait at a fuel station
  12. Find minimum missing positive number in a sequence
  13. Least-recently used cache (LRU)
  14. Count pairs of a given expected sum
  15. Binary heap (min-heap)
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  24. Leaky Bucket Rate Limiter
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  27. Longest increasing sub-sequence length
  28. Reverse first n elements of a queue
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  40. Balanced parentheses algorithm with tail-call recursion optimisation
  41. Reverse a String's words efficiently
  42. Count number of changes (manipulations) needed to make an anagram with an efficient foldLeft
  43. Count passing cars
  44. Count dist intersections
  45. Establish execution order from dependencies
  46. Counting inversions of a sequence (array) using a Merge Sort
  47. Longest common prefix of strings
  48. Check if an array is a palindrome
  49. Compute missing ranges
  50. Check a directed graph has a routing between two nodes (depth-first search)
  51. Compute nth row of Pascal's triangle
  52. Run-length encoding (RLE) Decoder
  53. Check if a number is a palindrome
  54. In a range of numbers, count the numbers divisible by a specific integer
  55. Merge intervals
  56. Compute minimum number of Fibonacci numbers to reach sum
  57. Find the longest palindrome within a string
  58. Find the index of a substring ('indexOf')
  59. Reshape a matrix
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Explore the 22 most useful Scala concepts

To save you going through various tutorials, we cherry-picked the most useful Scala concepts in a consistent form.

  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. Variance
  21. View
  22. Zip

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