LeetCode: Roman to Integer

Problem difficulty: Easy

Problem description

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 ones 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
Explanation: III = 3.

Example 2:

Input: s = "LVIII"
Output: 58
Explanation: L = 50, V= 5, III = 3.

Example 3:

Input: s = "MCMXCIV"
Output: 1994
Explanation: M = 1000, CM = 900, XC = 90 and IV = 4.

Solution approach

For this problem, we want to convert a Roman numeral string to an integer.

• A smaller number can only appear before a larger number. For example, IX represents 9 (1 for I and 10 for X), but IIX is invalid. The correct representation for 8 is VIII.

Here’s a simple approach to solve this:

1. Create a hash table to map each Roman numeral character to its corresponding integer value (e.g., I -> 1, V -> 5, etc.)
2. Initialize a variable result to store the final integer value.
3. If the current character (mapStr[i]) is less than the next character (mapStr[i+1]), it represents a subtraction case (like IX). In this case, subtract the value of the current character from the next character and add the difference to the result.
4. Otherwise, simply add the value of the current character to the result.

class Solution:
    def romanToInt(self, s: str) -> int:
        mapNum = {
                    "I": 1,
                    "V": 5,
                    "X": 10,
                    "L": 50,
                    "C": 100,
                    "D": 500,
                    "M": 1000
                }
        num = 0
        i = 0
        while i != len(s):
            if i+1 < len(s) and mapNum[s[i]] < mapNum[s[i+1]]:
                num += mapNum[s[i+1]] - mapNum[s[i]]
                i +=2
            else:
                num += mapNum[s[i]]
                i += 1
        return num

Source code: https://github.com/hongquan080799/leetcode/tree/master/roman_to_integer