Integer to Roman

http://www.leetcode.com/onlinejudge
http://en.wikipedia.org/wiki/Roman_numerals

Given an integer, convert it to a roman numeral.

Input is guaranteed to be within the range from 1 to 3999.

/*Passed both small and large tests**/
class Solution {
    string buildDigit(string ivx, int num){//num [0~9]
        string result("");
        if (!num) return result;//0
        else if (num <= 3){//1,2,3
            for(;num>0;--num){
                result.push_back(ivx[0]);
            }
        }
        else if (num == 4 || num == 9){//4,9
            result.push_back(ivx[0]);
            result.push_back(ivx[num/4]);//1,or 2
        }
        else{//5,6,7,8
            result.push_back(ivx[1]);
            for(; num > 5; --num){
                result.push_back(ivx[0]);
            }
        }
        return result;

    }
public:
    string intToRoman(int num) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function    
        vector<string> one(3,"");
        one[0]="IVX";
        one[1]="XLC";
        one[2]="CDM";
        string result("");
        for(int ii = 0; ii < 3 && num>0; ++ii){
            int digit = num%10;
            num /= 10;
            result = buildDigit(one[ii], digit)+result;
        }
        for(;num>0;--num){
            result = one[2][2] + result;
        }
        return result;
    }
};