17. 电话号码的字母组合

输入："23"
输出：["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].

class Solution {
    public List<String> letterCombinations(String digits) {
        List<String> result = new ArrayList<String>();
        if(digits.length() == 0) {
            return result;
        }
        Map<Character, String> map = new HashMap<>();
        map.put('2', "abc");
        map.put('3', "def");
        map.put('4', "ghi");
        map.put('5', "jkl");
        map.put('6', "mno");
        map.put('7', "pqrs");
        map.put('8', "tuv");
        map.put('9', "wxyz");

        backTrack(digits, map, result, new StringBuffer(), 0);
        return result;
    }

    private void backTrack(String digits, Map<Character, String> map,List<String> combinations, StringBuffer combination, int index){
        if(digits.length() == index){
            combinations.add(combination.toString());
        } else {
            String letter = map.get(digits.charAt(index));
            for(int i = 0; i < letter.length(); i++) {
                combination.append(letter.charAt(i));
                backTrack(digits, map, combinations, combination, index + 1);
                combination.deleteCharAt(index);
            }
        }
    }
}