17、电话号码的数字组合

class Solution {
    private final Map<Character, String> phoneMap = new HashMap<Character, String>();

    public void init() {
        phoneMap.put('2', "abc");
        phoneMap.put('3', "def");
        phoneMap.put('4', "ghi");
        phoneMap.put('5', "jkl");
        phoneMap.put('6', "mno");
        phoneMap.put('7', "pqrs");
        phoneMap.put('8', "tuv");
        phoneMap.put('9', "wxyz");
    }

    public List<String> letterCombinations(String str) {
        List<String> res = new ArrayList<String>();
        if (str.length() == 0) {
            return res;
        }
        init();
        backtrack(res, phoneMap, str, 0, new StringBuffer());
        return res;
    }

    public void backtrack(List<String> res, Map<Character, String> phoneMap, String str, int index, StringBuffer sb) {
        if (index == str.length()) {
            res.add(sb.toString());
            return;
        }
        String letters = phoneMap.get(str.charAt(index));
        int lettersCount = letters.length();
        for (int i = 0; i < lettersCount; i++) {
            sb.append(letters.charAt(i));
            backtrack(res, phoneMap, str, index + 1, sb);
            sb.deleteCharAt(index);
        }
    }
}