Find Special Substring of Length K

Description

You are given a string s and an integer k.

Determine if there exists a substring of length exactly k in s that satisfies the following conditions:

- The substring consists of only one distinct character (e.g., "aaa" or "bbb").

- If there is a character immediately before the substring, it must be different from the character in the substring.

- If there is a character immediately after the substring, it must also be different from the character in the substring.

Return true if such a substring exists. Otherwise, return false.