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[]={1,4,5,9,10,40,50,90,100,400,500,900,1000};
        string str="";
        for(int i=12;i>=0;--i){
            while(num>=b[i]){
                str+=a[i];
                num=num-b[i];
            }
        }
        return str;
    }
};