mysql常用查询：group by，左连接，子查询，having where

class Solution {
public:
    vector<string> letterCombinations(string digits) {
        vector<string> ret;
        int len = digits.length();
        
        string n2s[] = {
            "",
            "",
            "abc",
            "def",
            "ghi",
            "jkl",
            "mno",
            "pqrs",
            "tuv",
            "wxyz"
        };
        
        // 1
        /*
        vector<int> curpos(len, 0);
        while (true) {
            ret.push_back("");
            int ind = ret.size() - 1;
            for (int i = 0; i < len; i++) {
                int nind = digits[i] - ‘0‘;
                if (n2s[nind].length() > 0) {
                    ret[ind].push_back(n2s[nind][curpos[i]]);
                }
            }
            
            int k = len - 1;
            while (k >= 0) {
                int nind = digits[k] - ‘0‘;
                if (curpos[k] < n2s[nind].length() - 1) {
                    curpos[k]++;
                    break;
                }
                
                curpos[k--] = 0;
            }
            
            if (k < 0) break;
        }
        */
        
        // 2
        string curs = "";
        genLetterCombination(ret, curs, digits, n2s, 0);
        
        return ret;
    }

private:
    void genLetterCombination(vector<string> &ret, string &curs, string &d, string n2s[], int cpos) {
        if (cpos == d.length()) {
            ret.push_back(curs);
            return;
        }
        
        int dind = d[cpos] - ‘0‘;
        for (int i = 0; i < n2s[dind].length(); i++) {
            curs.push_back(n2s[dind][i]);
            genLetterCombination(ret, curs, d, n2s, cpos + 1);
            curs.pop_back();
        }
    }
};