First Double Letter

MEDIUMString Processing7 marks

Read a word and find the first pair of adjacent matching letters.

The comparison is case-insensitive. Output the matching letter in uppercase. If there is no adjacent pair, output None.

Input: One word. Output: The first repeated adjacent letter in uppercase, or None.

Example:

Input:  book
Output: O

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Sample Test Cases

Test 1: Double O

Inputs: book

Expected: O

Test 2: First double is ss

Inputs: Mississippi

Expected: S

Test 3: No adjacent repeat

Inputs: abcde

Expected: None