1class Solution {2    private final String[] letters = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};3    public List<String> letterCombinations(String digits) {4        List<String> result = new ArrayList<>();5        if (digits.isEmpty()) return result;6        search(digits, 0, new StringBuilder(), result);7        return result;8    }9    private void search(String digits, int index, StringBuilder path, List<String> result) {10        if (index == digits.length()) {11            result.add(path.toString());12            return;13        }14        String choices = letters[digits.charAt(index) - '0'];15        for (int i = 0; i < choices.length(); i++) {16            path.append(choices.charAt(i));17            search(digits, index + 1, path, result);18            path.deleteCharAt(path.length() - 1);19        }20    }21}