1.思路:

   1.1: 将减法得到的数字对应成单独的数字，见上图（来自力扣官网题解）;
   1.2: 问题转换为用13中面值不同的钞票拼成一个整数；
   1.3：可以采用贪心法，每次尽量尝试用较大面值的钞票去拼凑，这样可以使用的钞票数量最少

2.代码

class Solution {
public:
    string intToRoman(int num) {
     int value[]={1000,900,500,400,100,90,50,40,10,9,5,4,1};
     string reps[]={"M","CM","D","CD","C","XC","L","XL","X","IX","V","IV","I"};
     string res;
     for(int i=0;i<13;i++)//贪心，从面值大的开始拼凑
     {
         while(num>=value[i])//注意是>=!!!!!!
         {
            num-=value[i];
            res+=reps[i];
         }
     }
     return res;
    }
};