roman to integer

class Solution {
public:
    int romanToInt(string s) {
        if(s.length() == 0)
        {
            return 0;
        }

        map<char, int> num;
        num['I'] = 1;
        num['V'] = 5;
        num['X'] = 10;
        num['L'] = 50;
        num['C'] = 100;
        num['D'] = 500;
        num['M'] = 1000;

        int result = num[s[s.length()-1]];
        for(int i = s.length()-2; i>=0; i--)
        {
            if(num[s[i]] < num[s[i+1]])
            {
                result -= num[s[i]];
            }
            else
            {
                result += num[s[i]];
            }
        }

        return result;
    }
};