list/src/main/java/util/Combinations.java

package util;

import java.util.Arrays;
import java.util.HashSet;
import java.util.stream.IntStream;

public class Combinations {
	private static HashSet<HashSet<Integer>> result = new HashSet<>();


	/* arr[]  ---> Input Array
		data[] ---> Temporary array to store current combination
		start & end ---> Staring and Ending indexes in arr[]
		index  ---> Current index in data[]
		r ---> Size of a combination to be printed */
	static void combinationUtil(int arr[], Integer data[], int start, int end, int index, int combinationLength) {
		// Current combination is ready to be printed, print it
		if (index == combinationLength) {
			result.add(new HashSet<>(Arrays.asList(data)));
			return;
		}

		// replace index with all possible elements. The condition
		// "end-i+1 >= r-index" makes sure that including one element
		// at index will make a combination with remaining elements
		// at remaining positions
		for (int i = start; i <= end && end - i + 1 >= combinationLength - index; i++) {
			data[index] = arr[i];
			combinationUtil(arr, data, i + 1, end, index + 1, combinationLength);
		}
	}

	public static HashSet<HashSet<Integer>> generateIndices(int maxNOfIndices) {
		result = new HashSet<>();
		int[] arr = IntStream.range(1, maxNOfIndices).toArray();
		for (int i = 1; i < maxNOfIndices - 1; i++) {
			// A temporary array to store all combination one by one
			combinationUtil(arr, new Integer[i], 0, arr.length - 1, 0, i);
		}

		// also add an empty one for X.... (all of this type)
		result.add(new HashSet<>());

		return result;
	}
}