Integer to Roman [LEETCODE]

Given an integer, convert it to a roman numeral.

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

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class Solution {
public:
    string intToRoman(int num) {
        // Note: The Solution object is instantiated only once and is reused by each test case.    
        string roman_char[] = {"M","CM","D","CD","C","XC","L","XL","X","IX","V","IV","III","II","I"};
        int roman_value[]   ={1000,900, 500,400, 100, 90,  50, 40,  10,  9,   5,  4,    3,   2,   1};
        string result = "";
        for(int i = 0; i < 15; i++){
            if(num >= roman_value[i]){
                num -= roman_value[i];
                result += roman_char[i];
                i = -1;
            }
        }
        return result;
    }
};