[LeetCode]Letter Combinations of a Phone Number

class Solution {
//static declare and definition with local scope
public:
	void letterCombinations_aux(int step, string& path, vector<string>& ans, const string& digits)
	{
		//pay attention to this kind of statement
		const static string strT[10] = {"","","abc","def","ghi","jkl","mno","qprs","tuv","wxyz"};
		if(step == digits.size())
		{
			ans.push_back(path);
			return;
		}
		for(int i = 0; i < strT[digits[step]-'0'].size(); ++i)
		{
			path.push_back(strT[digits[step]-'0'][i]);
			letterCombinations_aux(step+1, path, ans, digits);
			path.pop_back();
		}
	}
    vector<string> letterCombinations(string digits) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        string path;
		vector<string> ans;
		int step = 0;
		letterCombinations_aux(step, path, ans, digits);
		return ans;
    }
};