题目描述：给定一个仅包含数字 2-9 的字符串，返回所有它能表示的字母组合。答案可以按 任意顺序 返回。
给出数字到字母的映射如下（与电话按键相同）。注意 1 不对应任何字母。

示例 1：
输入：digits = "23"
输出：["ad","ae","af","bd","be","bf","cd","ce","cf"]

示例 2：
输入：digits = ""
输出：[]

示例 3：
输入：digits = "2"
输出：["a","b","c"]

class Solution {
public:
    void backtrack(vector<string> &result,vector<string> phone,string digits,int num,string &temp){
        if(num==digits.size()){
            result.push_back(temp);
            return;
        }
        int n=digits[num]-'0';
        for(int i=0;i<phone[n].size();i++){
            temp.push_back(phone[n][i]);
            backtrack(result,phone,digits,num+1,temp);
            temp.pop_back();
        }
    }
    vector<string> letterCombinations(string digits) {
        vector<string> result;
        if(digits.size()==0) return result;
        string temp;
        vector<string> phone={"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
        backtrack(result,phone,digits,0,temp);
        return result;
    }
};