class QuickSort
{
	public static void main( String[] args )
	{
		int[] array = { 8, 6, 9, 13, 1, 17, 6, 19, 2, 18, 16 };
		qsort( array );
	}

	public static void qsort( int[] array )
	{
		sortSubArray( array, 0, array.length-1 );
	}

	private static void sortSubArray( int[] array, int l, int u )
	{
		if( l >= u ) 
			return;

		// find the pivot value
		// use the array[l] point as the pivot
		final int pivot = array[l];

		int m = l;

		// scan the sub-array from l to u,
		//      partitioning it to 2 groups.
		// move values < p to positions <=m in the array
		// keep values >= p to positions >m in the array
		for( int i=l+1; i <= u; i++ ) {
			if( array[i] < pivot ) {
				m++;
				swap( array, m, i );
			}
		}

		// move the pivot to the correct position
		swap(  array, l, m );

		// recursively sort the partitions
		sortSubArray( array, l, m-1 );
		sortSubArray( array, m+1, u );
	}

	private static void swap( int[] array, int i, int j )
	{
		int temp = array[i];
		array[i] = array[j];
		array[j] = temp;
	}
}