[LC] 159. Longest Substring with At Most Two Distinct Characters

Given a string s , find the length of the longest substring t that contains at most 2 distinct characters.

Example 1:

Input: "eceba"
Output: 3
Explanation: tis "ece" which its length is 3.

Example 2:

Input: "ccaabbb"
Output: 5
Explanation: tis "aabbb" which its length is 5.

Time: O(N)
Space: O(N)

 1 class Solution:
 2     def lengthOfLongestSubstringTwoDistinct(self, s: str) -> int:
 3         res = 0
 4         start, end, count = 0, 0, 0
 5         my_dict = {}
 6         while end < len(s):
 7             end_char = s[end]
 8             end_freq = my_dict.get(end_char, 0)
 9             my_dict[end_char] = end_freq + 1
10             if end_freq == 0:
11                 count += 1
12             end += 1
13             // cannot use count <= 2 to get res b/c end will run before start
14             while count > 2:
15                 start_char = s[start]
16                 start_freq = my_dict[start_char]
17                 my_dict[start_char] = start_freq - 1
18                 if start_freq == 1:
19                     count -= 1
20                 start += 1
21             res = max(res, end - start)
22         return res