Permutations for given number

Permutations.java

package com.and.test;

import java.util.ArrayList;
import java.util.List;

public class Solution {

	/**
	 * Publicly exposed method, that wraps method with the actual algorithm.
	 * This is also to conform with the rules as outlined by the coding
	 * challenge - "NB Please do not change the name of the method or the way
	 * the functions is called"
	 *
	 * @param input The String to be permuted
	 * @return String containing permutations of input
	 */

	public static String solution(String input) throws NumberFormatException {
		return solution("", input).toString();
	}

	/**
	 * Recursive algorithm for finding permutations of given string. Works by
	 * keeping one character fixed and then calculating permutations of the
	 * other characters.
	 *
	 * This method will produce duplicate permutations in the case that the
	 * input contains the same value twice - Eg 1123 will produce 2113 twice.
	 * Returns n! permutations
	 *
	 * @param prefix Stores result
	 * @param input  The String to be permuted
	 * @return List containing permutations of input
	 */
	private static List<String> solution(String prefix, String input) {
		List<String> permutations = new ArrayList<>();

		if (input.isEmpty()) {
			permutations.add(prefix);
		} else {
			for (int i = 0; i < input.length(); i++) {
				permutations.addAll(solution(prefix + input.charAt(i), input.substring(0, i) + input.substring(i + 1)));

			}
		}

		return permutations;
	}

	/**
	 * sanitizeString will sanitize; in this case meaning removing all non-digit
	 * characters for the given input.
	 *
	 * \D - matching all non-digits
	 *
	 * @param input The String to be sanitized
	 * @return The resulting string
	 */
	private static String sanitizeString(String input) {
		return input.replaceAll("\\D", "");
	}

	public static void main(String args[]) {
		/**
		 * Just a quick way to generate a random string,
		 * Requires import java.util.UUID;
		 * String input = UUID.randomUUID().toString().substring(4,8);
		 */

		String input = "1234";
		input = sanitizeString(input);
		System.out.println(solution(input));
	}

}