Count Substrings That Differ by One Character

For each starting position pair (i in s, j in t), expand outward from a mismatch point to count valid substrings. The count is (left_matches + 1) * (right_matches + 1), where left_matches and right_matches are consecutive matching characters on each side of the single differing position.

Edge case with a single character.

Since no characters match between s and t, every possible substring pair of the same length will differ by exactly one character. The valid pairs are: (a,d), (b,e), (c,f), (ab,de), (bc,ef), (ac,de), (ab,ef), (abc,def), totaling 9 pairs where each differs by exactly one character.