leetcode 17 电话号码的字母组合

解题关键点：用递归方法

class Solution {
public:
    vector<string> mapping = { "abc","def","ghi","jkl",
                            "mno","pqrs","tuv","wxyz"};
    
    void combinations(string digits, int idx, string str, vector<string>& result)
    {
        if (idx == digits.size())
        {
            result.push_back(str);
            return;
        }
        string chars = mapping[digits[idx]-'2'];
        for (int i = 0; i < chars.size(); i++)
        {
            combinations(digits,idx+1,str+chars[i],result);
        }
    }
public:
    vector<string> letterCombinations(string digits) {
        vector<string> result;
        if (digits.size() == 0) return result;

        combinations(digits,0,"",result);

        return result;
    }
};