Leetcode Integer to Roman

Given an integer, convert it to a roman numeral.

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

int key[]={1000, 900, 500, 400, 100,90, 50, 40, 10, 9, 5, 4, 1};
string romans[]={"M", "CM", "D", "CD", "C", "XC", "L", "XL", "X", "IX", "V", "IV", "I"};

class Solution {
public:
    string intToRoman(int num) {
        for(int i =0 ; i < 13; ++ i){
            if(key[i]<= num) return romans[i]+intToRoman(num-key[i]);
        }
        return "";
    }
};