Longest Substring with Two Distinct Characters

Given a string s, your task is to compute the length of the longest substring that contains at most two distinct characters. In other words, find the maximum length L such that there exists a substring s[i..j] with at most two unique characters, and j - i + 1 = L.

Note: If the input string is empty, the answer is 0.

The method you choose should be efficient (for instance, using a sliding window approach, which runs in O(n) time).

The mathematical condition for a valid substring can be expressed in \( \LaTeX \) as follows:

[ |{ s[k] : i \le k \le j }| \le 2 ]

inputFormat

The input consists of a single line containing the string s. This string may be empty.

outputFormat

Output a single integer representing the length of the longest substring containing at most two distinct characters.

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