Cassido's weekly newsletter interview question of the week 2/24/19

phonePad.java

import java.util.*;
/**
 * Created by kylel95 on 3/1/19.
 */
public class phonePad {
    public static void main(String [] args){
        String s = "123456";
        perm1(makeString(s));
    }

    /**
     * @param s Phone number to permute
     * @return Permuted string based on number to letters mapping on a phone pad
     */
    public static String makeString(String s){
        HashMap<Character, String> phone = new HashMap<>();
        StringBuilder sb = new StringBuilder();
        phone.put('0', "");
        phone.put('1', "");
        phone.put('2', "ABC");
        phone.put('3', "DEF");
        phone.put('4', "GHI");
        phone.put('5', "JKL");
        phone.put('6', "MNO");
        phone.put('7', "PQRS");
        phone.put('8', "TUV");
        phone.put('9', "WXYZ");
        for(int i = 0; i < s.length(); i++){
            sb.append(phone.get(s.charAt(i)));
        }
        return sb.toString();
    }


    /**
     * Helper permutation method to calculate permutations of a string
     * //https://introcs.cs.princeton.edu/java/23recursion/Permutations.java.html
     // print n! permutation of the characters of the string s (in order)
     * @param s String to permute
     */
    public  static void perm1(String s) { perm1("", s); }
    /**
     * Calculates all s.length! permutation of a string
     * @param prefix prefix being appended to from the permuted string s
     * @param s String to be permuted
     */
    private static void perm1(String prefix, String s) {
        int n = s.length();
        if (n == 0) System.out.println(prefix);
        else {
            for (int i = 0; i < n; i++)
                perm1(prefix + s.charAt(i), s.substring(0, i) + s.substring(i+1, n));
        }

    }
}