letter-combinations-of-a-phone-number

题目：

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.

程序：

class Solution {public:      vector<string> res;    string tmp;    map<int, string> num2alp = {{2, "abc"}, {3, "def"}, {4, "ghi"}, {5, "jkl"}, {6, "mno"}, {7, "pqrs"}, {8, "tuv"}, {9,"wxyz"}};    vector<string> letterCombinations(string digits) {        if(digits.empty()){            res.push_back(tmp);            return res;        }        else            getStr(digits, 0, tmp);        return res;    }    void getStr(string digits, int idx, string tmp){        if(idx == digits.size())            res.push_back(tmp);        int im = digits[idx] - '0';        for(int i = 0; i < num2alp[im].size(); ++i){            getStr(digits, idx + 1, tmp + num2alp[im][i]);        }    }};

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