romanToInteger.py

# DATE 28/08/2025

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

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.

SOLUTION
Build a hashmap of the roman numerals and their integer values. Iterate through the string, checking if the current numeral is less than the next numeral. If it is, subtract its value from the total. If not, add its value to the total.
- Time complexity: O(n)
- Space complexity: O(1)
"""

class Solution:
    def romanToInt(self, s: str) -> int:
        hashMap = {"I": 1, "V": 5, "X": 10, "L": 50, "C": 100, "D": 500, "M": 1000}
        total = 0
        for ind in range(len(s)):
            if (ind+1 <= len(s) - 1) and (hashMap.get(s[ind]) < hashMap.get(s[ind+1])):
               total -= hashMap.get(s[ind])
            else: 
                total += hashMap.get(s[ind])
        return total