Given a string s, return the length of the longest substring that contains at most two distinct characters. Example 1: Input: s = "eceba" Output: 3 Explanation: The substring is "ece" which its length is 3. Example 2: Input: s = "ccaabbb" Output: 5 Explanation: The substring is "aabbb" which its length is 5. Constraints: 1 <= s.length <= 104 s consists of English letters.