Roman to Integer

Given a roman numeral, convert it to an integer.

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

class Solution {
public:
int romanToInt(string s) {
int res = 0;
for (int i = 0; i < s.size()-1; ++i){
if (convert2Number(s[i]) < convert2Number(s[i + 1]))
res -= convert2Number(s[i]);
else
res += convert2Number(s[i]);
}

res += convert2Number(s[s.size()-1]);
return res;
}

int convert2Number(char c){
switch (c){
case 'I':
return 1;
case 'V':
return 5;
case 'X':
return 10;
case 'L':
return 50;
case 'C':
return 100;
case 'D':
return 500;
case 'M':
return 1000;
default:
return 0;
}
}
};