17.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"].

Note:
Although the above answer is in lexicographical order, your answer could be in any order you want.

思路：DFS，help函数生成第index个数字对应的字母。如果index=digits.length()，就将生成的字符串加入result数组。否则对digits[index]所有可能的字符加入elem，递归调用help函数。

1. class Solution {
   private:
       string temp[10]={"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
       vector<string> result;
   public:
       void help(string digits,string elem,int index){
           if(index==digits.length()){
               result.push_back(elem);
               return;
           }
           int val = digits[index]-'0';
           for(int i=0;i<temp[val].length();i++){
               help(digits,elem+temp[val][i],index+1);
           }
       
       }
       vector<string> letterCombinations(string digits) {
           if(digits.length()==0)
               return result;
           help(digits,"",0);
           return result;
       }
   };