LetterCombinationsofaPhoneNumber.java

package com.fishercoder.solutions;

import java.util.*;

/**
 * Given a digit string, return all possible letter combinations that the number could represent.

 A mapping of digit to letters (just like on the telephone buttons) is given below.

 Input:Digit string "23"
 Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].
 Note:
 Although the above answer is in lexicographical order, your answer could be in any order you want.
 */

public class LetterCombinationsofaPhoneNumber {

    public List<String> letterCombinations(String digits) {
        String[] digits2Letters = new String[]{"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};

        List<String> result = new ArrayList();
        if(digits.length() == 0) return result;

        result.add("");//this line is important, otherwise result is empty and Java will default it to an empty String
        for(int i = 0; i < digits.length(); i++){
            result = combine(digits2Letters[digits.charAt(i)-'0'], result);
        }

        return result;
    }

    List<String> combine(String letters, List<String> result){
        List<String> newResult = new ArrayList();

        for(int i = 0; i < letters.length(); i++){//the order of the two for loops doesn't matter, you could swap them and it still works.
            for(String str : result){
                newResult.add(str + letters.charAt(i));
            }
        }
        return newResult;
    }
}