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.

 

思路：回溯

class Solution {public:    vector
     
       letterCombinations(string digits) {        vector
      
        lux(10);        lux[2] = "abc"; lux[3] = "def"; lux[4] = "ghi"; lux[5] = "jkl";        lux[6] = "mno"; lux[7] = "pqrs"; lux[8] = "tuv"; lux[9] = "wxyz";        vector
       
         ans;        if(digits.empty())        {            ans.push_back("");            return ans;        }        string X = digits;        vector
        
          S(digits.length());        int k = 0;        while(k >= 0)        {            while(k >= 0 && S[k] < lux[digits[k] - '0'].length())            {                X[k] = lux[digits[k] - '0'][S[k]++];                if(k < digits.length() - 1)                {                    k++;                }                else                {                    ans.push_back(X); //当判断条件是 （长度 - 1）  时不需要 k-- 因为最后一位数字需要循环                }            }            S[k] = 0;            k--;        }        if(ans.empty())        {            ans.push_back("");            return ans;        }        return ans;    }};