DAY 26: Roman to Integer: leetcode : python3

 

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.

Constraints:

• 1 <= s.length <= 15
• s contains only the characters ('I', 'V', 'X', 'L', 'C', 'D', 'M').
• It is guaranteed that s is a valid roman numeral in the range [1, 3999].

SOLUTION

class Solution():

    def romanToInt(self, s: str) -> int:

        """

        :type s: str

        :rtype: int

        """

        roman: dict[str, int] = { 'I': 1,

        'V':5,

        'X':10,

        'L':50,

        'C':100,

        'D':500,

        'M':1000 }

       

        sol=0

        for a,b in zip(s,s[1:]):

            if roman[a]<roman[b]:

                sol -=roman[a]

            else:

                sol +=roman[a]

        return sol + roman[s[-1]]

Time Complexity: O(n)

Space Complexity:O(1)

How it works:

• First, the code creates a dictionary called roman that maps Roman numerals to their corresponding values. For example, 'I' maps to 1, 'V' maps to 5, and so on.
• Next, the code creates a variable called sol that will store the final result.
• Then, the code loops through the string s two characters at a time. For each pair of characters, the code checks if the first character is less than the second character.
• If the first character is less than the second character, the code subtracts the value of the first character from sol. For example, if the current pair of characters is 'IV', the code would subtract 1 from sol because 'I' is less than 'V'.
• If the first character is not less than the second character, the code adds the value of the first character to sol. For example, if the current pair of characters is 'MD', the code would add 1000 to sol because 'M' is not less than 'D'.
• Finally, the code returns the value of sol.