Roman to Integer

Given a roman numeral, convert it to an integer.

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

Solution:

int romanToInt(string s) {
    unordered_map<char, int> T = { { 'I' , 1 }, { 'V' , 5 }, { 'X' , 10 }, { 'L' , 50 }, { 'C' , 100 }, { 'D' , 500 }, { 'M' , 1000 } };
    int sum = T[s.back()];

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

    return sum;
}