17. *的字母组合

给定一个仅包含数字 2-9 的字符串，返回所有它能表示的字母组合。答案可以按 任意顺序 返回。

给出数字到字母的映射如下（与电话按键相同）。注意 1 不对应任何字母。

示例 1：

输入：digits = "23"
输出：["ad","ae","af","bd","be","bf","cd","ce","cf"]
示例 2：

输入：digits = ""
输出：[]
示例 3：

输入：digits = "2"
输出：["a","b","c"]

class Solution {
    public List<String> letterCombinations(String digits) {
        List<String> list = new ArrayList<>();
        String[] s = new String[digits.length()];
        int M = digits.length();
        if (s.length == 0)
        {
            return list;
        } 
        for (int i = 0; i < digits.length();i++)
        {
            switch (digits.charAt(i))
            {
                case '2': s[i] = "abc"; break;
                case '3': s[i] = "def"; break;
                case '4': s[i] = "ghi"; break;
                case '5': s[i] = "jkl"; break;
                case '6': s[i] = "mno"; break;
                case '7': s[i] = "pqrs"; break;
                case '8': s[i] = "tuv"; break;
                case '9': s[i] = "wxyz"; break;
             }
        }
        list = getStringWithFor(s, 0 ,list, "");
        return list;
    }

    private static List<String> getStringWithFor(String[] s, int i, List<String> list, String stemp)
    {
        if (i < s.length - 1)
        {
            for (int j = 0; j < s[i].length(); j++)
            {
                list = getStringWithFor(s, i + 1, list, stemp + s[i].charAt(j));
            }
            i++;
        }else{
            for (int j = 0; j < s[i].length(); j++)
            {
                list.add(stemp + s[i].charAt(j));
            }
        }
        return list;
    }
}