Leetcode 567. Permutation in String

class Solution:
    def checkInclusion(self, s1: str, s2: str) -> bool:
        n = len(s1)
        stat = collections.Counter(s1)
        stat['length'] = n
        subs = collections.defaultdict(int)
        for index, ch in enumerate(s2):
            if ch in stat:
                subs[ch] += 1
                subs['length'] += 1
            else:
                subs = collections.defaultdict(int)
                continue
            if subs['length'] == stat['length']:
                if stat == subs:
                    return True
                subs[s2[index - n + 1]] -= 1
                subs['length'] -= 1
        return False