Letter Combinations of a Phone Number

Given a string containing digits from 2-9 inclusive, 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. Note that 1 does not map to any letters.

Example:

Input: "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.

class Solution {
    public String[] helper(char c) {
        if (c == '2')
            return new String[] { "a", "b", "c" };
        else if (c == '3')
            return new String[] { "d", "e", "f" };
        else if (c == '4')
            return new String[] { "g", "h", "i" };
        else if (c == '5')
            return new String[] { "j", "k", "l" };
        else if (c == '6')
            return new String[] { "m", "n", "o" };
        else if (c == '7')
            return new String[] { "p", "q", "r", "s" };
        else if (c == '8')
            return new String[] { "t", "u", "v" };
        else if (c == '9')
            return new String[] { "w", "x", "y", "z" };
        else
            return new String[0];
    }

    public List<String> letterCombinations(String digits) {
        List<String> ans = new ArrayList<>();
        if (digits.length() == 0)
            return ans;
        backTrack(digits, ans, new StringBuilder(), 0);
        return ans;
    }

    public void backTrack(String digits, List<String> ans, StringBuilder str, int start) {
        if (start == digits.length()) {
            ans.add(str.toString());
            return;
        }
        String[] chars = helper(digits.charAt(start));
        for (int i = 0; i < chars.length; i++) {
            str.append(chars[i]);
            backTrack(digits, ans, str, start + 1);
            str.deleteCharAt(str.length() - 1);
        }
    }
}