12 Integer to Roman(int转罗马数字Medium)

题目意思：1-3999转罗马数字

思路：从大往小减

　　ps:这题有点蛋疼

 class Solution {
 public:
     string intToRoman(int num) {
         string a[]={"I","IV","V","IX","X","XL","L","XC","C","CD","D","CM","M"};
         int b[]={,,,,,,,,,,,,};
         string str="";
         for(int i=;i>=;--i){
             while(num>=b[i]){
                 str+=a[i];
                 num=num-b[i];
             }
         }
         return str;
     }
 };