leetcode 13 Roman to Integer C++

int romanToInt(string s) {
int a[26] = {0};
a['I'-'A'] = 1;
a['V'-'A'] = 5;
a['X'-'A'] = 10;
a['L'-'A'] = 50;
a['C'-'A'] = 100;
a['D'-'A'] = 500;
a['M'-'A'] = 1000;

int sum = 0;
int max = 1;
for (int i = s.size() - 1; i >= 0; i--) {
if (a[s[i]-'A'] >= max) {
max = a[s[i]-'A'];
sum += a[s[i]-'A'];
}else{
sum -= a[s[i]-'A'];
}
}
return sum;
}