Quick Sort (Lomuto)

Choose the last item as a pivot, partition smaller values to its left, then recurse on the two sides.

Algorithm

The checked-in replay follows the same small input and final output across all 21 DSA books, so this Ruby DSA implementation can be compared directly with the other languages.

Basic Implementation

basic.rb

def partition(arr, low, high)
	pivot = arr[high]
	i = low - 1
	(low...high).each do |j|
		if arr[j] <= pivot
			i += 1
			arr[i], arr[j] = arr[j], arr[i]
		end
	end
	arr[i + 1], arr[high] = arr[high], arr[i + 1]
	i + 1
end

def quick_sort(arr, low, high)
	if low < high
		pivot_index = partition(arr, low, high)
		quick_sort(arr, low, pivot_index - 1)
		quick_sort(arr, pivot_index + 1, high)
	end
end

arr = [4, 1, 5, 2, 3]
quick_sort(arr, 0, arr.length - 1)
puts arr.inspect

Complexity

Time: O(n^2) worst, O(n log n) average
Space: O(log n) average call stack
Stable: no

Implementation notes

Keep the explicit algorithmic steps instead of calling a standard-library sort. The replay is meant to expose comparisons, movement, and recursion.
The implementation is intentionally compact for learning and replay, not a production sorting utility.

pivot The final element is moved to the boundary between smaller and larger values.

partition One scan rearranges the current range before the recursive calls.