Returns a list of combined words given a sequence of numbers and a Number-Letter Mapping

playNumberLetterCombination.java

 //Keypad Number to Letter Mapping
 //2-9 corresponds to letters on the phone keypad and the same index of the array
    private static String[] keyPadMapping = {"0", "1", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv" ,"wxyz"};
   
    public static List<String> playNumberLetterCombination(String number){

        List<String> combinedWords = new ArrayList<>();

        //empty or wrong input
        if(number.isEmpty() || !number.matches("\\d+")) return combinedWords;

        combination("", number, 0, combinedWords);

        return combinedWords;
    }

    private static void combination(String combo, String number, int numberIndex, List<String> combinedWords)
    {
        //break recursion here
        if(numberIndex >= number.length()){
           combinedWords.add(combo);
           return;
        }

        //get number
        int index = Character.digit(number.charAt(numberIndex), 10);

        //get corresponding letters
        String letters = keyPadMapping[index]; //retrieve number mapping

        for(int i = 0; i < letters.length(); i++){
            combination(combo+letters.charAt(i), number, numberIndex + 1, combinedWords);
        }
    }

Hello @deedee47, congratulations 🎉 your solution has been selected as one of the winning solutions in Week 8 of #AlgorithmFridays.

Your solution is clean, readable and passed the tests. I like your use of regex on line 10 and recursion for solving this problem - that was neat!

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