public static < E extends Comparable < ? super E >>
     void sort(E[] a, int i, int j) {
      if (i < j - 1) {

       int p = partition(a, i, j);
       sort(a, i, p);
       sort(a, p + 1, j);

      }
     }

    public static < E extends Comparable < ? super E >>
     int partition(E[] a, int i, int j) {
      E pivot = a[j - 1];
      int low = j - 1;
      int pow = i;
      while (pow < low)
      // a:| <= x | ? | > x |x|
      //    ^      ^   ^       ^
      //    i     pow low      j  x = pivot;
      {

       if (a[pow].compareTo(pivot) > 0) {
        swap(a, pow, --low);
       } else {
        pow++;
       }

      }
      swap(a, pow, j - 1);
      return pow;
     }
    public static < E extends Comparable < ? super E >>
     void swap(E[] a, int i, int j) {

      E temp = a[i];
      a[i] = a[j];
      a[j] = temp;
     }