leetcode-----17. *的字母组合

• 链接：https://leetcode-cn.com/problems/letter-combinations-of-a-phone-number/

代码

class Solution {
    private String letters[] = {" ", "", "abc", "def", "ghi", "jkl", "mno", 
    "pqrs", "tuv", "wxyz"};
    private List<String> ans;
    public List<String> letterCombinations(String digits) {
        ans = new ArrayList<String>();
        if (digits.equals("")) return ans;
        help(digits, 0, "");
        return ans;
    }

    private void help(String digits, int index, String s) {
        if (index == digits.length()) {
            ans.add(s);
            return ;
        }
        Character c = digits.charAt(index);
        String l = letters[c - '0'];
        for (int i = 0; i < l.length(); ++i) {
            help(digits, index + 1, s + l.charAt(i));
        }
    }
}