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"].

class Solution {
public:
    vector<string> letterCombinations(string digits) {
        //digits传递的是数字比如"23"
        vector<string> res;
        //这一步好难 如果为空，则返回空
        if(digits=="") return {};
        string tmp(digits.size(),' ');
        dfs(res,tmp,digits,0);
        return res;
        
    }
    //pos 表示添加的第几个！
    void dfs(vector<string> &res,string &tmp,string digits,int pos)
    {
       string table[]={" ","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
   
        //找不到问题出在哪里
        if(pos==digits.size())
        {
            res.push_back(tmp);
            return;
        }
        for(int i=0;i<table[digits[pos]-'0'].size();i++)
        {
            tmp[pos]=table[digits[pos]-'0'][i];
            dfs(res,tmp,digits,pos+1);
        }
       
    }
};

1: 组合问题。

2 pos传递的是组合的位置

3 当输入的是空集怎么处理

4 tmp[0],tmp[1]实现了覆盖。不需要退出选的值