Find K-Length Substrings With No Repeated

lcpy gen -n 1100   # by problem number
lcpy gen -s find_k_length_substrings_with_no_repeated_characters   # by problem name

Input: s = "havefunonleetcode", k = 5
Output: 6
Explanation: There are 6 substrings they are: 'havef','avefu','vefun','efuno','etcod','tcode'.

Input: s = "home", k = 5
Output: 0
Explanation: Notice k can be larger than the length of s. In this case, it is not possible to find any substring.

from collections import Counter


class Solution:
    # Time: O(n)
    # Space: O(1) - at most 26 distinct letters
    def num_k_len_substr_no_repeats(self, s: str, k: int) -> int:
        if k > len(s):
            return 0
        cnt = Counter(s[:k])
        ans = int(len(cnt) == k)
        for i in range(k, len(s)):
            cnt[s[i]] += 1
            cnt[s[i - k]] -= 1
            if cnt[s[i - k]] == 0:
                cnt.pop(s[i - k])
            ans += int(len(cnt) == k)
        return ans