Substrings of Size Three with Distinct Characters — Python

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

import collections


class Solution(object):
    def countGoodSubstrings(self, s):
        """
        :type s: str
        :rtype: int
        """
        K = 3

        result = 0
        count = collections.Counter()
        for i in xrange(len(s)):
            if i >= K:
                count[s[i-K]] -= 1
                if not count[s[i-K]]:
                    del count[s[i-K]]
            count[s[i]] += 1
            if len(count) == K:
                result += 1
        return result