cs1 combinations

KeyPadCombinations.java

/*
 * Question: Generate all the possible combinations of the letters on the telephone keypads. Recursively.
 * Basically, the product of N sets. 
 * e.g given 2,3. Valid combinations include (A,D),(A,E),(B,D)...
 */
public class KeyPadCombinations {
	/*
	 * Parameters 
	 * numbers = the key pad #s index = the position of a letter for
	 * each number level = the position of the current number
	 */
	public static void combo(int[] numbers, int[] index, int level) {
		// if we are at the last number
		if (level == numbers.length - 1) {
			// for every letter of the number on this level
			for (int j = 0; j < getLetters(numbers[level]).length; j++) {
				// print the letter value of the first number up to number at LEVEL.
				for (int k = numbers.length - 1; k > 0; k--) {
					System.out.print(getLetters(numbers[level - k])[index[level - k]]);
				}
				// print the letter of the current (last) number
				System.out.print(getLetters(numbers[level])[j]);
				// create new line
				System.out.println();
			}
		} else {
			// if not the last number, save the position of the letter in loop.
			for (int i = 0; i < getLetters(numbers[level]).length; i++) {
				index[level] = i;
				// recursive call on the next number.
				combo(numbers, index, level + 1);
			}
		}

	}
	
	//returns a set of letters in the num set
	public static String[] getLetters(int num) {

		String[] keys = new String[4];
		String[] two = { "A", "B", "C" };
		String[] three = { "D", "E", "F" };
		String[] four = { "G", "H", "I" };
		String[] five = { "J", "K", "L" };
		String[] six = { "M", "N", "O" };
		String[] seven = { "P", "Q", "R", "S" };
		String[] eight = { "T", "U", "V" };
		String[] nine = { "W", "X", "Y", "Z" };

		switch (num) {
			case 2: keys = two; break;
			case 3: keys = three; break;
			case 4: keys = four; break;
			case 5: keys = five; break;
			case 6: keys = six; break;
			case 7: keys = seven; break;
			case 8: keys = eight; break;
			case 9: keys = nine; break;
			default: System.out.println("Invalid Number"); break;
		}

		return keys;
	}

	public static void main(String[] args) {
		int[] a = { 2, 9, 5, 7, 4, 2, 3 };
		int[] index = new int[10];
		combo(a, index, 0);
	}

}