Longest Substring With At Most K Distinct Chars

from collections import defaultdict
class Solution:
    def lengthOfLongestSubstringKDistinct(self, s: 'str', k: 'int') -> 'int':
        n = len(s)
        if k == 0 or n == 0:
            return 0

        # sliding window left and right pointers
        left, right = 0, 0
        # hashmap character -> its rightmost position
        # in the sliding window
        hashmap = defaultdict()

        max_len = 1

        while right < n:
            # add new character and move right pointer
            hashmap[s[right]] = right
            right += 1

            # slidewindow contains 3 characters
            if len(hashmap) == k + 1:
                # delete the leftmost character
                del_idx = min(hashmap.values())
                del hashmap[s[del_idx]]
                # move left pointer of the slidewindow
                left = del_idx + 1

            max_len = max(max_len, right - left)

        return max_len