Number of Valid Words in a Sentence — Python

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

class Solution(object):
    def countValidWords(self, sentence):
        """
        :type sentence: str
        :rtype: int
        """
        result = token = hyphen = 0
        for i in xrange(len(sentence)+1):
            if i == len(sentence) or sentence[i] == ' ':
                if token == 1:
                    result += 1
                token = hyphen = 0
                continue
            if sentence[i].isdigit() or \
               (sentence[i] in "!.," and not (i == len(sentence)-1 or sentence[i+1] == ' ')) or \
               (sentence[i] == '-' and not (hyphen == 0 and 0 < i < len(sentence)-1 and sentence[i-1].isalpha() and sentence[i+1].isalpha())):
                token = -1
                continue
            if token == 0:
                token = 1
            if sentence[i] == '-':
                hyphen = 1
        return result