[LeetCode17]Letter Combinations of a Phone Number

Given a digit string, return all possible letter combinations that the number could represent.
A mapping of digit to letters (just like on the telephone buttons) is given below.

Input:Digit string "23"
Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].

Note: Although the above answer is in lexicographical order, your answer could be in any order you want. Analysis:
The idea is to use recursion. DFS slove it.
Java

public class LetterCombinationPhoneNum {
	ArrayList<String> res;
	StringBuffer seq;
	public void getletterComba(String digits, int level, String[] trans){
		if(level == digits.length()){
			res.add(seq.toString());
			return;
		}
		int digit = digits.charAt(level)-48;
		for(int i=0;i<trans[digit].length();i++){
			seq.append(trans[digit].charAt(i));
			getletterComba(digits, level+1, trans);
			seq.deleteCharAt(level);
		}
	}
	public List<String> letterCombinations(String digits) {
		res = new ArrayList<>();
		seq = new StringBuffer();
        String []trans = {"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
        getletterComba(digits, 0, trans);
        return res;
    }
}

c++

public:
    void getLetterCombination(string &seq,
                          string &digits,
                          int level,
                          vector<string> &result,string trans[])
{
    if(level == digits.size()){
        result.push_back(seq);
        return;
    }
    int group = digits[level] - 48;
    for(int i=0; i<trans[group].size(); i++){
        seq.push_back(trans[group][i]);
        getLetterCombination(seq,digits,level+1,result,trans);
        seq.resize(seq.size()-1);
    }
}
vector<string> letterCombinations(string digits) {
    string trans[] = {"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
    vector<string> result;
    string seq;
    getLetterCombination(seq,digits,0,result,trans);
    return result;
}