159. Longest-Substring-with-At-Most-Two-Characters

from collections import defaultdict

class Solution:
    """
    @param s: a string
    @return: the length of the longest substring T that contains at most 2 distinct characters
    """
    def length_of_longest_substring_two_distinct(self, s: str) -> int:
        # Write your code here
        n = len(s)
        charCount = defaultdict(int)
        cntChar = 0
        i = 0
        res = 0
        for j in range(n):
            charCount[s[j]] += 1
            if charCount[s[j]] == 1:
                cntChar += 1
            while cntChar > 2:
                charCount[s[i]] -= 1
                if charCount[s[i]] == 0:
                    cntChar -= 1
                i += 1

            res = max(res, j - i + 1)
        return res