Integer to Roman

Given an integer, convert it to a roman numeral.

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

此題找規律模擬操作，分析1-10的整數是如何構造出羅馬數字的，然後再依葫蘆畫瓢，注意細節的處理。

class Solution {
public:
    string IntToR(int k, char mask[])
    {
        string Res = "";
        if(k <= 3)
        {
            for(int i = 0; i < k; ++i)
            {
                Res.push_back(mask[0]);
            }
        }
        else if(k == 4)
        {
            Res.push_back(mask[0]);
            Res.push_back(mask[1]);
        }
        else if(k <= 8)
        {
            Res.push_back(mask[1]);
            for(int i = 0; i < k - 5; ++i)
            {
                Res.push_back(mask[0]);
            }
        }
        else if(k == 9)
        {
            Res.push_back(mask[0]);
            Res.push_back(mask[2]);
        }
        return Res;
    }
    
    string intToRoman(int num) {
          char mask[] = {'I', 'V', 'X', 'L', 'C', 'D', 'M'};
          int i = 6;
          int factor = 1000;
          string Res = "";
          while(num != 0)
          {
              int k = num / factor;
              num = num % factor;
              factor /= 10;
              Res += IntToR(k, &mask[i]);
              i -= 2;
          }
          return Res;
    }
};