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 Java DSA implementation can be compared directly with the other languages.

Basic Implementation

Basic.java

class Basic {
	public static void main(String[] args) {
		int[] arr = new int[] { 4, 1, 5, 2, 3 };
		quickSort(arr, 0, arr.length - 1);
		printArray(arr);
	}

	static void printArray(int[] arr) {
		System.out.print("[");
		for (int i = 0; i < arr.length; i++) {
			if (i > 0) System.out.print(", ");
			System.out.print(arr[i]);
		}
		System.out.println("]");
	}
	static int partition(int[] arr, int low, int high) {
		int pivot = arr[high];
		int i = low - 1;
		for (int j = low; j < high; j++) {
			if (arr[j] <= pivot) {
				i++;
				int tmp = arr[i]; arr[i] = arr[j]; arr[j] = tmp;
			}
		}
		int tmp = arr[i + 1]; arr[i + 1] = arr[high]; arr[high] = tmp;
		return i + 1;
	}

	static void quickSort(int[] arr, int low, int high) {
		if (low < high) {
			int pivotIndex = partition(arr, low, high);
			quickSort(arr, low, pivotIndex - 1);
			quickSort(arr, pivotIndex + 1, high);
		}
	}
}

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.