public class QuickSort {
    public static void main(String[] args) {
        int[] arr = {5, 1, 7, 3, 1, 6, 9, 4, 2};

        quickSort(arr, 0, arr.length - 1);

        for (int i : arr) {
            System.out.print(i + "\t");
        }
    }

    private static void quickSort(int[] arr, int leftIndex, int rightIndex) {
        if (leftIndex > rightIndex) {
            return;
        }
        int l = leftIndex;
        int r = rightIndex;
        int key = arr[l];

        while (l < r) {
            while (l < r && arr[r] >= key) {
                r--;
            }
            arr[l] = arr[r];
            while (l < r && arr[l] <= key) {
                l++;
            }
            arr[r] = arr[l];
        }
        arr[l] = key;
        quickSort(arr, leftIndex, l - 1);
        quickSort(arr, r + 1, rightIndex);
    }
}

Q.E.D.