Longest Substring with At Most K Distinct Characters — Python

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

class Solution(object):
    def lengthOfLongestSubstringKDistinct(self, s, k):
        """
        :type s: str
        :type k: int
        :rtype: int
        """
        longest, start, distinct_count, visited = 0, 0, 0, [0 for _ in xrange(256)]
        for i, char in enumerate(s):
            if visited[ord(char)] == 0:
                distinct_count += 1
            visited[ord(char)] += 1
            while distinct_count > k:
                visited[ord(s[start])] -= 1
                if visited[ord(s[start])] == 0:
                    distinct_count -= 1
                start += 1
            longest = max(longest, i - start + 1)
        return longest


# Time:  O(n)
# Space: O(1)
from collections import Counter


class Solution2(object):
    def lengthOfLongestSubstringKDistinct(self, s, k):
        """
        :type s: str
        :type k: int
        :rtype: int
        """
        counter = Counter()
        left, max_length = 0, 0
        for right, char in enumerate(s):
            counter[char] += 1
            while len(counter) > k:
                counter[s[left]] -= 1
                if counter[s[left]] == 0:
                    del counter[s[left]]
                left += 1
            max_length = max(max_length, right-left+1)
        return max_length