[Leetcode] Roman to Integer

class Solution {
public:
int romanToInt(string s) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
int res = 0;
int prev = 1001;
int cur = 0;
int len = s.size();
for (int i = 0; i < len; ++i)
{
cur = map(s[i]);
if (cur <= prev)
res += cur;
else
{
res += cur;
res -= 2 * prev;
}
prev = cur;
}

return res;
}

int map(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;
}
}

};