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;    }};