Assume that removing exactly one character from s1 can yield s2.
Produce all positions in s1 where deleting the character leaves precisely s2.
When identical characters allow more than one qualifying removal, report every such index.
Notes
If the relationship is not guaranteed by the prompt, begin by checking that len(s1) is one greater than len(s2).
For small input limits, you can use an O(n²) baseline that removes each character in turn and tests the resulting string.
To achieve O(n), locate the earliest differing position and extend through the matching-character streak in s1; each position in that streak may represent a valid deletion.
Consecutive duplicates are the key edge case: with s1 = "bbbc" and s2 = "bbc", the answer must contain all three possible positions, [0, 1, 2], because removing any one of the three b characters gives "bbc".
Preparation
First write the exhaustive implementation as a reference for correctness, and afterward implement the O(n) scan that handles the duplicate run.
Cover removals at the beginning, end, and interior, along with a repeated-character segment that produces multiple valid indices.