17. 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.

给出一个字符串，代表数字按键顺序，求出所有可能的字母按键顺序。这里使用队列来实现。具体实现见代码：

代码：

class Solution {public:    vector<string> letterCombinations(string digits) {        vector<string> res;        if(digits.empty() || digits == "") return res;        queue<string> q;        q.push("");        for(int i = 0; i < digits.size(); ++i) {            string chars = getChars(digits[i]);            int qs = q.size();            for(int j = 0; j < qs; ++j) {                string s = q.front();                q.pop();                for(auto c:chars) {                    q.push(s+c);                }            }        }        while(!q.empty()) {            res.push_back(q.front());            q.pop();        }        return res;    }private:    string getChars(char d) {        vector<string> map{"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};        return map[d-'0'];    }};