**LeetCode #159 - Longest Substring with At Most Two Distinct Characters**\nGiven a string s, return the length of the longest substring that contains at most two distinct characters. This problem requires an understanding of the sliding window technique to efficiently find the maximum length of the substring under the specified constraint.\n**Function Signature:** `def length_of_longest_substring_two_distinct(s: str) -> int:`\n**Example 1:**\nInput: `s = "eceba"`\nOutput: `3`\nExplanation: The substring is "ece" which contains two distinct characters.\n**Example 2:**\nInput: `s = "aa"`\nOutput: `2`\nExplanation: The substring is "aa" which contains one distinct character.\n**Constraints:**\n- `1 <= s.length <= 10^5`\n- `s` consists of English letters, digits, symbols, and spaces.