Number of Matching Subsequences

class Solution(object):
    def numMatchingSubseq(self, s, words):
        """
        :type s: str
        :type words: List[str]
        :rtype: int
        """

        n = len(s)

        alphabets = {chr(97+i): [] for i in range(26)}
        for i, e in enumerate(s):
            alphabets[e].append(i)

        ans = 0
        for word in words:
            flag = True
            start = 0
            m = len(word)

            for indx, letter in enumerate(word):
                end = n-m+indx
                indexes = alphabets[letter]

                if len(indexes) == 0 or indexes[-1] < start or end < indexes[0]:
                    flag = False
                    break
                if start <= indexes[0]:
                    start = indexes[0] + 1
                    continue

                l = 1
                r = len(indexes) - 1
                mid = (l+r)//2
                while(True):
                    if l > r:
                        start = indexes[l]+ 1
                        break
                    if indexes[mid] == start:
                        start = indexes[mid]+ 1
                        break
                    if start <indexes[mid]:
                        r = mid- 1
                    else:
                        l= mid+ 1
                    mid = (l+r)//2

            if flag == True:
                ans+= 1

        return ans