For example,If the roman is "DCXXI",D has no last_number,so we plus the D=500.C is small than its last_number "D",so we plus C=100.go on and on,we get 621.

When the roman is "CDXXI",the D is bigger than its last_number C,we need to minus the C.But we have plus the C before,so we should minus the C twice.

If you are Chinese,there is a Chinese website show the difference between roman and integer.

http://www.cnblogs.com/dosxp/archive/2008/08/13/1266781.html

```
roman_dict={"M":1000,"D":500,"C":100,"L":50,"X":10,"V":5,"I":1}
last_num = 0
result_num = 0
for i in s:
if roman_dict[i] <= last_num:
result_num = result_num + roman_dict[i]
else:
result_num = result_num -2*last_num + roman_dict[i]
last_num = roman_dict[i]
return result_num
```