187. Repeated DNA Sequences 
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Problem
The DNA sequence is composed of a series of nucleotides abbreviated as 'A', 'C', 'G', and 'T'.
- For example,
"ACGAATTCCG"is a DNA sequence.
When studying DNA, it is useful to identify repeated sequences within the DNA.
Given a string s that represents a DNA sequence, return all the 10-letter-long sequences (substrings) that occur more than once in a DNA molecule. You may return the answer in any order.
Example 1:
Input: s = "AAAAACCCCCAAAAACCCCCCAAAAAGGGTTT" Output: ["AAAAACCCCC","CCCCCAAAAA"]
Example 2:
Input: s = "AAAAAAAAAAAAA" Output: ["AAAAAAAAAA"]
Constraints:
1 <= s.length <= 105s[i]is either'A','C','G', or'T'.