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.

这道题看起来十分弱小，吃掉它

class Solution {public:	vector
     
       letterCombinations(string digits) {		// Start typing your C/C++ solution below		// DO NOT write int main() function		vector
      
        ret;		if(digits.length()==0){		    ret.push_back("");		    return ret;		}		map
       
         m;		m['2']="abc";		m['3']="def";		m['4']="ghi";		m['5']="jkl";		m['6']="mno";		m['7']="pqrs";		m['8']="tuv";		m['9']="wxyz";		m['0']=" ";		m['*']="+";		vector
        
          ptr;		ptr.resize(digits.length());		for(int i=0;i
         
          =digits.length()){					ret.push_back(rs);					i--;					rs.resize(i);				}				else{					ptr[i]=0;				}			}		}		return ret;	}};