quicksort_M

import java.util.Arrays;

public class Main {

    public static void main(String[] args) {
        int[] a = new int[]{1000, 10, 7, 8, 9, 30, 900, 1, 5, 6, 20};
        quickSort(a,0,a.length-1);
        System.out.println(Arrays.toString(a));

    }

    public static void quickSort(int[] arr,int l,int h){
        if (l < h){
            int n = h-l+1;
            int med = kthSmallest(arr,l,h,n/2);
            int p = partition(arr,l,h,med);

            quickSort(arr,l,p-1);
            quickSort(arr,p+1,h);
        }
    }

    private static int partition(int[] arr, int l, int r, int x) {
        int i;
        for (i=l; i<r; i++)
            if (arr[i] == x)
                break;
        swap(arr,i,r);
        i = l;
        for (int j = l; j <= r - 1; j++)
        {
            if (arr[j] <= x)
            {
                swap(arr,i,j);
                i++;
            }
        }
        swap(arr,i,r);
        return i;
    }

    private static void swap(int[] arr, int i, int r) {
        int t = arr[i];
        arr[i] = arr[r];
        arr[r] = t;
    }

    private static int kthSmallest(int[] arr, int l, int r, int k) {
        if (k > 0 && k <= r - l + 1)
        {
            int n = r-l+1;
            int i;
            int[] median = new int[(n+4)/5];
            for (i=0; i<n/5; i++)
                median[i] = findMedian(arr,i*5, 5);
            {
                median[i] = findMedian(arr,i*5, n%5);
                i++;
            }

            int medOfMed = (i == 1)? median[i-1]:
                    kthSmallest(median, 0, i-1, i/2);

            int pos = partition(arr, l, r, medOfMed);

            if (pos-l == k-1)
                return arr[pos];
            if (pos-l > k-1)
                return kthSmallest(arr, l, pos-1, k);

            return kthSmallest(arr, pos+1, r, k-pos+l-1);
        }

        return Integer.MAX_VALUE;
    }

    private static int findMedian(int[] arr, int start, int len) {
        Arrays.sort(arr,start,start+len);
        return arr[start+len/2];
    }
}