import java.util.Random;

public class Partition {

	// like http://www.cplusplus.com/reference/algorithm/partition/
	// but additionally places separator in the end of the first group
	public static int partition(int[] a, int fromInclusive, int toExclusive, int separatorIndex) {
		int i = fromInclusive;
		int j = toExclusive - 1;
		if (i >= j) return j;
		int separator = a[separatorIndex];
		swap(a, i++, separatorIndex);
		while (i <= j) {
			while (i <= j && a[i] < separator)
				++i;
			while (i <= j && a[j] > separator)
				--j;
			if (i >= j)
				break;
			swap(a, i++, j--);
		}
		swap(a, j, fromInclusive);
		return j;
	}

	static void swap(int[] a, int i, int j) {
		int t = a[j];
		a[j] = a[i];
		a[i] = t;
	}

	// Random test
	public static void main(String[] args) {
		Random rnd = new Random(1);
		for (int step = 0; step < 100_000; step++) {
			int n = rnd.nextInt(10) + 1;
			int[] a = rnd.ints(n, 0, 10).toArray();
			for (int i = 0; i < n; i++) {
				for (int j = i; j < n; j++) {
					for (int k = i; k <= j; k++) {
						int[] b = a.clone();
						check(b, partition(b, i, j + 1, k), i, j);
					}
				}
			}
		}
	}

	static void check(int[] a, int k, int lo, int hi) {
		if (k < lo || k > hi)
			throw new RuntimeException();
		for (int i = lo; i <= k; i++)
			for (int j = k; j <= hi; j++)
				if (a[i] > a[j])
					throw new RuntimeException();
	}
}