Letter Combinations of a Phone Number

class Solution {/*Straightforward, and easy**/
private:
vector<string> rep;
vector<string> result;
void build(const string & digits, int digitIndex, string ss)
{
int  num = digits[digitIndex]-'0';
if (digitIndex + 1 == digits.length())
{
for(int buttonIndex = 0; buttonIndex < rep[num].length(); ++buttonIndex)
{
result.push_back(ss+rep[num][buttonIndex]);
}
return;
}
for(int buttonIndex = 0; buttonIndex < rep[num].length(); ++buttonIndex)
{
build(digits, digitIndex+1,ss+rep[num][buttonIndex]);
}
return;
}
public:
vector<string> letterCombinations(string digits) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
if (digits.empty()) return vector<string>(1,"");
result.clear();
rep.resize(10);
rep[2]="abc";
rep[3]="def";
rep[4]="ghi";
rep[5]="jkl";
rep[6]="mno";
rep[7]="pqrs";
rep[8]="tuv";
rep[9]="wxyz";
build(digits, 0, "");
return result;
}
};