One edit distance: are two strings exactly one edit apart

Return whether two strings are exactly one edit apart, where an edit inserts one character, deletes one character or replaces one character. Equal strings are zero edits apart, so they return False. Clarify the edge cases before coding, then solve it in one pass.

Interface

Unless you agree on a different interface with the interviewer, assume:

• is_one_edit_distance(s: str, t: str) -> bool

Example

is_one_edit_distance("ab", "acb")     ->  True    # insert "c"
is_one_edit_distance("cab", "ad")     ->  False
is_one_edit_distance("1203", "1213")  ->  True    # replace "0" with "1"
is_one_edit_distance("abc", "abc")    ->  False   # zero edits
is_one_edit_distance("", "a")         ->  True
is_one_edit_distance("", "")          ->  False

Constraints

• 0 <= len(s), len(t) <= 10^5; any characters.
• Target O(n) time and O(1) extra space.

Clarifying questions

• Do equal strings count as one edit apart?
• Are empty strings allowed?
• Is the comparison case-sensitive?
• Exactly one edit, or at most one?

What a strong answer covers

• If lengths differ by more than one, return False; make s the shorter string.
• Walk both strings to the first mismatch: with equal lengths, the rest must match after skipping one character in each (replace); with lengths differing by one, the rest of the shorter must match the longer after skipping one character there (insert).
• If no mismatch is found, the answer is True only when the lengths differ by one.
• Comparing tails by index rather than slicing to keep O(1) space.
• Tests: both empty, one empty, the mismatch at the last character, equal strings.

Follow-ups

• At most k edits apart, efficiently for small k (banded edit distance).
• Return the edit itself (which operation, where).
• Check many words against a dictionary for one-edit neighbours.