LeetCode17. Letter Combinations of a Phone Number

题目链接：

https://leetcode.com/problems/letter-combinations-of-a-phone-number/

题目描述：

给一个数字字符串，和一个数字的映射表，找出所有的字母组合。顺序可任意。

Input:Digit string “23”
Output: [“ad”, “ae”, “af”, “bd”, “be”, “bf”, “cd”, “ce”, “cf”].

题目分析：

递归来解，对当前已经找到的字符串，递归剩下的字符串。

代码：

class Solution {
public:
    vector<string> s={ " ","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
    vector<string> res;
    int n;
    void fun(string ss,string digits,int idx){
      if (idx == n){
        res.push_back(ss);
        return;
      }
      int j = digits[idx] - '0';
      for (int i = 0; i<s[j].size(); i++){
            string sss = ss;
            ss += s[j][i];
            fun(ss, digits, idx + 1);
            ss = sss;
      }
    }
    vector<string> letterCombinations(string digits) {
        n=digits.size();
        if(n==0){
            return res;
        }
        string ss="";
        fun(ss,digits,0);
        return res;
    }
};