整数转罗马数字，13题的镜像问题，用贪心，每次找最大的值

string intToRoman(int num) {
        string symbol[] = {"M","CM","D","CD","C","XC","L","XL","X","IX","V","IV","I"};    
        int value[] = {1000,900,500,400,100,90,50,40,10,9,5,4,1}; 
        int i=0;
        string res = "";
        while(num!=0){
            if(num>=value[i]){
                res+=symbol[i];
                num-=value[i];
            }
            else i++;    
        }
        return res;
    }