Letter Combinations of a Phone Number

class Solution {
public:
    const vector<string> keyboard{"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
    
    vector<string> letterCombinations(string digits) {
        vector<string> result;
        if(digits.empty())
            return result;
        dfs(digits, 0, "", result);
        return result;
    }
    
    void dfs(const string &digits, int cur, string path, vector<string> &result)
    {
        if(digits.size() == cur)
        {
            result.push_back(path);
            return;
        }
        
        for(int i=0; i <keyboard[digits[cur]-‘0‘].size(); ++i)
            dfs(digits, cur+1, path+keyboard[digits[cur]-‘0‘][i], result);
    }
};