Working from back to front makes this problem easier. The only rule you need to know for roman numerals when working back to front is that if the current value is less than the last value, subtract it from your running total and continue on.

```
int getValue(char s){
switch(s){
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 1/0;
break;
}
}
int romanToInt(string s) {
int cur = 0;
int prev = 0;
int total = 0;
for(int i = s.size()-1; i>=0; i--){
cur = getValue(s.at(i));
if(cur < prev){
total = total - cur;
}
else{
total = total + cur;
}
prev = cur;
}
return total;
}
```