Leetcode: Letter Combinations of a Phone Number in C++

Given a digit string, return all possible letter combinations that the number could represent.
Solution:
vector<char> switchchar(char input)
    {
    vector<char> result;
switch(input)
{
case '2':
result.push_back('a');
result.push_back('b');
result.push_back('c');
break;
case '3':
result.push_back('d');
result.push_back('e');
result.push_back('f');
break;
case '4':
result.push_back('g');
result.push_back('h');
result.push_back('i');
break;
case '5':
result.push_back('j');
result.push_back('k');
result.push_back('l');
break;
case '6':
result.push_back('m');
result.push_back('n');
result.push_back('o');
break;
case '7':
result.push_back('p');
result.push_back('q');
result.push_back('r');
result.push_back('s');
break;
case '8':
result.push_back('t');
result.push_back('u');
result.push_back('v');
break;
case '9':
result.push_back('w');
result.push_back('x');
result.push_back('y');
result.push_back('z');
break;
   }
   return result;
    }
    void helper(string digits, string cur, int num, vector<string>& result )
    {
        if(num==digits.size())
        {
            result.push_back(cur);
        }
        char digit = digits[num];
        vector<char> option = switchchar(digit);
        for(int i=0;i<option.size();i++)
        {
            helper(digits, cur+option[i], num+1, result);
        }
    }
    vector<string> letterCombinations(string digits) {
        vector<string> result;
        string cur = "";
        helper(digits, cur, 0, result);
        return result;
    }