A DNA sequence consists of a series of nucleotides, each represented by one of the characters 'A', 'C', 'G', or 'T'.

Given a string s representing a DNA sequence, find and return all $10$-letter-long substrings that appear more than once within s. The result may be returned in any order.

Constraints:

• $1 \leq$ s.length $\leq 10^5$

• s[i] is one of 'A', 'C', 'G', or 'T'

The key idea is to use a sliding window of fixed size $10$ to scan through the DNA string and identify all $10$-letter substrings that occur more than once. We slide a window of length $10$ across the string, extracting each substring and tracking how many times it has been seen using a hash map (seen). Whenever a substring is encountered for exactly the second time, we add it to our result list. By only appending when the count equals $2$, we naturally avoid adding ...