[LeetCode] 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.

典型的dfs 或者说backtrace 思路

class Solution {
            map<char, string> m_maps;
            string m_digits;
    public:
       void backtrace(string & inStr, vector<string> & result)
//       void backtrace(string  inStr, vector<string> & result)
        {   
            if(inStr.size() == m_digits.size())
            {   
                result.push_back(inStr);
                return;
            }   
            size_t inSize = inStr.size(); 
            string charMap = m_maps[m_digits[inSize]];

            for(size_t i = 0; i < charMap.size(); i++)
            {   
                inStr += charMap[i];
                backtrace(inStr, result);
                inStr.erase(inStr.end() - 1); 
            }   
        }   

        vector<string> letterCombinations(string digits) {
            vector<string> result;
            string inStr;
                
            m_maps['2'] = "abc";
            m_maps['3'] = "def";
            m_maps['4'] = "ghi";
            m_maps['5'] = "jkl";
            m_maps['6'] = "mno";
            m_maps['7'] = "pqrs";
            m_maps['8'] = "tuv";
            m_maps['9'] = "wxyz";
            m_maps['0'] = " ";

            m_digits = digits;

            backtrace(inStr, result);

            return result;
        }
};

对于void backtrace(string & inStr, vector<string> & result) 和 void backtrace(string  inStr, vector<string> & result) 都是正确的，有无引用都可以。