LeetCode#13 Roman to Integer

//I(1),X(10),C(100),M(1000),V(5),L(50),D(500)
class Solution {
public:
int romanToInt(string s) {
int res=0;

for(int i = s.size()-1; i >= 0 ; i--)
{
switch(s[i])
{
case 'I':
res += res >= 5 ? -1:1; break;
case 'X':
res += res >= 50 ? -10:10; break;
case 'C':
res += res >= 500 ? -100:100; break;
case 'M':
res += 1000;break;
case 'V':
res += 5;break;
case 'L':
res += 50;break;
case 'D':
res += 500;break;
default:break;
}
}

return res;
}
};