Algorithm Interview: Convert Roman Numerals to Decimal

Hi, here's your problem today. This problem was recently asked by Twitter:

Given a Roman numeral, find the corresponding decimal value. Inputs will be between 1 and 3999.

Example:
Input: IX
Output: 9

Input: VII
Output: 7

Input: MCMIV
Output: 1904
Roman numerals are based on the following symbols:

I     1

IV    4

V     5

IX    9 

X     10

XL    40

L     50

XC    90

C     100

CD    400

D     500

CM    900

M     1000

Numbers are strings of these symbols in descending order. In some cases, subtractive notation is used to avoid repeated characters. The rules are as follows:
1.) I placed before V or X is one less, so 4 = IV (one less than 5), and 9 is IX (one less than 10)
2.) X placed before L or C indicates ten less, so 40 is XL (10 less than 50) and 90 is XC (10 less than 100).
3.) C placed before D or M indicates 100 less, so 400 is CD (100 less than 500), and 900 is CM (100 less than 1000).

class Solution():

  def romanToInt(self, s):

    # Fill this in.

    

n = 'MCMX'

print(Solution().romanToInt(n))

# 1910