17. Letter Combinations|Time:O(3^N*4^M)|Space: O(3^N*4^M)

// Problem: https://leetcode.com/problems/letter-combinations-of-a-phone-number/solution/
// Solution: https://www.youtube.com/watch?v=21OuwqIC56E

class Solution {
    public List<String> letterCombinations(String digits) {
        List<String> result = new ArrayList<String>();
        if(digits == null || digits.length() == 0) {
            return result;
        }

        String[] mappings = {
            "",
            "",
            "abc",
            "def",
            "ghi",
            "jkl",
            "mno",
            "pqrs",
            "tuv",
            "wxyz"
        };

        letterCombinationsRecursive(result, digits, "", 0, mappings);
        return result;
    }

    public void letterCombinationsRecursive(List<String> result, String digits, String current, int index, String[] mapping) {
        if(index  == digits.length()) {
            result.add(current);
            return;
        }

        String letters = mapping[digits.charAt(index) - '0'];
        for(int i = 0; i < letters.length(); i++) {
            letterCombinationsRecursive(result, digits, current + letters.charAt(i), index + 1, mapping);
        }
    }

}