leetcode 17.电话号码的字母组合

class Solution {
public:
    vector<string> result;
    string cur;
    const string letterMap[10]={
        "",
        "",
        "abc",
        "def",
        "ghi",
        "jkl",
        "mno",
        "pqrs",
        "tuv",
        "wxyz"
    };
    void getCur(string digits,int startIndex){
        if(startIndex==digits.size()){
            result.push_back(cur);
            return;
        }
        int digit=digits[startIndex]-'0';
        string letter = letterMap[digit];

        for(int i=0;i<letter.size();i++){
            cur.push_back(letter[i]);
            getCur(digits,startIndex+1);
            cur.pop_back();
        }
    }
    vector<string> letterCombinations(string digits) {
        result.clear();
        cur.clear();
        if(digits.size()==0){
            return result;
        }
        getCur(digits,0);
        return result;
    }
};