Method partition

Java/QuickSort.java:8–27

(int[] arr, int si, int ei)

public class QuickSort {

    public static int partition(int[] arr, int si, int ei) {
        int pid, piv = arr[si], count = 0, i = si;
        for (i = si + 1; i <= ei; i++) {
            if (arr[i] <= piv) {
                count++;
            }
        }
        pid = count + si;
        arr[si] = (arr[pid] + arr[si]) - (arr[pid] = arr[si]);
        while (si < pid && ei > pid) {
            if (arr[si] < piv) {
                si++;
            } else if (arr[ei] > piv) {
                ei--;
            } else {
                arr[si] = (arr[ei] + arr[si]) - (arr[ei--] = arr[si++]);
            }
        }
        return pid;
    }

    public static void quickSort(int[] arr, int si, int ei) {
        if (si < ei) {