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Roman to Integer

Problem​

Convert a Roman numeral string into its corresponding integer value. Roman numerals follow a set of rules, including subtractive notation for certain values (like IV for 4).

Approach​

The approach involves using a dictionary to map Roman numeral symbols to their integer values. We iterate through the input string, checking for the subtractive cases (where a smaller numeral precedes a larger one). If a subtractive case is found, we subtract the value of the current numeral and add the value of the next. Otherwise, we simply add the value of the current numeral to the total.

Solution​

"""
1. similar to previous problem, converting roman to int
2. list out dict as hashmap, iterate through the string 
3. to check for 4 and 9s case if next char is bigger than prev char it is either 4 or 9, hence subtract the present value and add next value
for eg:- 
IV :- sub 1 and add 5 so it results in 4
5. else add it to the ans int and return the ans

time comp:- o(n) -> no of char in str of len n
space comp:- o(1)
"""
class Solution:
    def romanToInt(self, s: str) -> int:
        conv_map = {
            "I": 1, "V": 5, "X": 10, "L": 50,
            "C": 100, "D": 500, "M": 1000
        }
        ans = 0
        n = len(s)
        for i in range(n):
            if i < n-1 and conv_map[s[i]] < conv_map[s[i+1]]:
                ans -= conv_map[s[i]]
            else:
                ans += conv_map[s[i]]
        return ans

Complexity​

  • Time: O(n) — We iterate through the string of Roman numerals once, where n is the length of the string.
  • Space: O(1) — We use a constant amount of extra space for the mapping dictionary and a few variables.