Valid Question Mark Strings

You are given a string s made of lowercase English letters and ? characters. Every ? must be replaced by a lowercase letter (each ? independently; different ? may get the same letter).

A completed string is valid when both conditions hold:

• No two adjacent characters are equal.
• The first character equals the last character. (For a string of length 1 this is automatically true.)

Letters that are already in s cannot be changed. Return the number of different valid strings that can be produced. Because the answer can be very large, return it modulo 10^9 + 7. If no valid string can be produced, return 0.