public class Main {
	// Given a string of alphabets and a number k, write a function to print all of the combinations of length k of these alphabets.
	// The string does not contain duplicate alphabets.
	// EXPECTED OUTPUT: abd, abs, abe, ads, ade, ase, bds, bde, bse, dse.
	public static void main(String[] args) {
		char[] alphabet = { 'a', 'b', 'd', 's', 'e' };
		int k = 3; // length of output string.

		System.out.println("EXPECTED OUTPUT: abd, abs, abe, ads, ade, ase, bds, bde, bse, dse");
		System.out.print("ACTUAL OUTPUT: ");
		printAllCombinations(alphabet, k);
	}

	// Base function used to call the recursive version of itself with given alphabet and length, k.
	private static void printAllCombinations(char[] alphabet, int k) {
		// Initially set the prefix to "" and n to 0, while passing alphabet and length, k to printAllCombinations.
		printAllCombinations(alphabet, "", 0, k);
	}

	// Recursive method used to print all possible strings of length k, given a set of characters.
	// However, use n to keep track of your current location in the array alphabets.
	private static void printAllCombinations(char[] alphabet, String prefix, int n, int k) {
		// Base case: k is 0,
		// print prefix.
		if (k == 0) {
			// Only print ', ' if n is less than the length of the current prefix.
			if (n > prefix.length()) {
				System.out.print(", ");
			}

			System.out.print(prefix);
			return;
		}

		// One by one add all characters
		// from set and recursively
		// call for k equals to k - 1.
		// However, loop only from n to length of the alphabet.
		for (int i = n; i < alphabet.length; i++) {
			// Next character of input added.
			String newPrefix = prefix + alphabet[i];

			// k is decreased and n is increased, because
			// we have added a new character.
			printAllCombinations(alphabet, newPrefix, i + 1, k - 1);
		}
	}
}