"""
Roman numerals are represented by seven different symbols: I, V 
X, L, C, D and M.

Symbol       Value
I             1
V             5
X             10
L             50
C             100
D             500
M             1000

For example, 2 is written as II in Roman numeral, just two 
one's added together. 12 is written as XII, which is simply X + 
II. The number 27 is written as XXVII, which is XX + V + II.

Roman numerals are usually written largest to smallest from 
left to right. However, the numeral for four is not IIII. 
Instead, the number four is written as IV. Because the one is 
before the five we subtract it making four. The same principle 
applies to the number nine, which is written as IX. There are 
six instances where subtraction is used:

I can be placed before V (5) and X (10) to make 4 and 9. 
X can be placed before L (50) and C (100) to make 40 and 90. 
C can be placed before D (500) and M (1000) to make 400 and 900.
Given a roman numeral, convert it to an integer.

Example 1:

Input: s = "III"
Output: 3
"""


class Solution:
    def romanToInt(self, s: str) -> int:
        roman = { 'I': 1, 'V': 5, 'X': 10, 'L': 50, 'C': 100, 
        'D': 500, 'M': 1000 }
                
        previous, number = -1, 0
        
        for c in s:
          current = roman[c]

          if previous != -1 and current > previous:
              number -= previous
              number += current - previous
          else:
              number += current
          previous = current
                
        return number