遞迴思想，字串逐步減短

#include <iostream>
#include <vector>
#include <algorithm>
#include <math.h>
#include <stdio.h>
#include <string>
using namespace std;

string A[10] = {"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};

class Solution {
public:
    vector<string> letterCombinations(string digits) {
        vector<string> res;
        return helper(res,digits);
    }
    
    
    vector<string> helper(vector<string> V, string s)
    {
        if(s=="")
        {
            return V;
        }
        else 
        {
            vector<string> ret;
            int num = s[0] - '0';
            string s1 = A[num];
			if(V.size()==0)
			{
				for(int i = 0;i<s1.size();i++)
				{
					string s = "";
					s.append(1,s1[i]);
					ret.push_back(s);
				}
			}
			else
			{
				for(int i = 0;i<V.size();i++)
				{
					for(int j = 0;j<s1.size();j++)
					{
						string t_s = V[i];
						t_s.append(1,s1[j]);
						ret.push_back(t_s);
					}
				}
			}
			return helper(ret, s.substr(1,s.size()-1));
        }
    }
    
};