Leetcode 159. Longest Substring with At Most Two Distinct Characters

[locked]

# Time:  O(n^2)
# Space: O(1)
#
# Given a string, find the length of the longest substring T 
# that contains at most 2 distinct characters.
# 
# For example, Given s = "eceba",
# 
# T is "ece" which its length is 3.
#

class Solution:
    # @param s, a string
    # @return an integer
    def lengthOfLongestSubstringTwoDistinct(self, s):
        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 > 2:
                visited[ord(s[start])] -= 1
                if visited[ord(s[start])] == 0:
                    distinct_count -= 1
                start += 1
  
            longest = max(longest, i - start + 1)
        return longest