Longest substring without three repeats in a row

Given a string s, return the length of the longest substring of s in which no character appears three or more times in a row. Call such a substring semi-alternating: runs of one or two equal characters are fine, but a run of three is not.

Implement:

def longest_semi_alternating(s: str) -> int

A substring is a contiguous slice of s. Return 0 for the empty string.

Constraints

• 0 <= len(s) <= 2 * 10^5
• s may contain any characters (letters, digits, symbols), not only two letters.
• Aim for O(n) time and O(1) extra space.

Example 1

s = "kkkhhkkkhh"
output: 6

The triple kkk appears twice. The longest stretch that avoids both is "kkhhkk" (indices 1 to 6).

Example 2

s = "ppqqpp"
output: 6

Every run has length at most two, so the whole string counts.

Interviewers often follow up by asking for the substring itself, a general limit of k repeats, or the number of valid substrings.