Check if Word Equals Summation of Two Words — Python

# Time:  O(n)
# Space: O(1)

class Solution(object):
    def isSumEqual(self, firstWord, secondWord, targetWord):
        """
        :type firstWord: str
        :type secondWord: str
        :type targetWord: str
        :rtype: bool
        """
        def stoi(s):
            result = 0
            for c in s:
                result = result*10 + ord(c)-ord('a')
            return result
        
        return stoi(firstWord) + stoi(secondWord) == stoi(targetWord)