[LeetCode] 13. Roman to Integer

Roman to Integer

Link to original Problem on LeetCode

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, two is written as II in Roman numeral, just two one’s added together. Twelve is written as, XII, which is simply X + II. The number twenty seven is written as XXVIIi, 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. Input is guaranteed to be within the range from 1 to 3999.

Example 1:

Input: “III” Output: 3 Example 2:

Input: “IV” Output: 4 Example 3:

Input: “IX” Output: 9 Example 4:

Input: “LVIII” Output: 58 Explanation: L = 50, V= 5, III = 3. Example 5:

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

Company: Yahoo, Google, Amazon, Microsoft

Solution

Time Complexity: O(n)

Space Complexity: O(1) # Since the dictionary only takes a constant space

Solution(object):
    def romanToInt(self, s):
        """
        :type s: str
        :rtype: int
        """
        # If we check the Roman number, there are basically 2 things which is happening
        # 1. Number correponding to the last letter is always added
        # 2. Except point 1. if the number corresponding to a letter is less than the number 
        #    corresponding to the letter next to that, then it will be subtracted, else added
        romanToIntDict = {'I': 1, 'V': 5, 'X': 10, 'L': 50, 'C': 100, 'D': 500, 'M': 1000}
        result = 0
        
        for i in range(len(s) - 1):
            # Using 2.
            if romanToIntDict[s[i]] < romanToIntDict[s[i + 1]]:
                result += -romanToIntDict[s[i]]
            else:
                result += romanToIntDict[s[i]]
        
        # Using 1.
        result += romanToIntDict[s[-1]]
        
        return result