Leetcode 13. Roman to Integer

class Solution {
public:
    int romanToInt(string s) {
        map<char, int> values;
        values['I'] = 1;
        values['V'] = 5;
        values['X'] = 10;
        values['L'] = 50;
        values['C'] = 100;
        values['D'] = 500;
        values['M'] = 1000;
        int value = 0;
        for(int i = 0; i < s.length() - 1; i++) {
            if(values[s[i]] < values[s[i + 1]]) {
                value -= values[s[i]];
            }
            else {
                value += values[s[i]];
            }
        }
        value += values[s[s.length() - 1]];
        return value;
    }
};