import java.util.function.Predicate;

public class Calculations {
	public static void main(String[] args) {
		char[] chars = {'a', 'b', 'c', 'd'};
		// поиск bba
		calculate(chars, 3, i -> i[0] == 1 && i[1] == 1 && i[2] == 0);
	}
	static void calculate(char[] a, int k, Predicate<int[]> decider) {
		int n = a.length;
		if (k < 1 || k > n)
			throw new IllegalArgumentException("Forbidden");
		int[] indexes = new int[n];
		int total = (int) Math.pow(n, k);
		while (total-- > 0) {
			for (int i = 0; i < n - (n - k) ; i++)
				System.out.print(a[indexes[i]]);
			System.out.println();
			if (decider.test(indexes) )
				break;
			for (int i=0; i < n; i++) {
				if (indexes[i] >= n - 1) {
					indexes[i] = 0;
				} else {
					indexes[i]++;
					break;
				}
			}
		}
	}
}