Repeated DNA Sequences

Problem Description

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.

Key Insights

• The DNA sequence consists of four nucleotides: 'A', 'C', 'G', and 'T'.
• We need to identify and return substrings of length 10 that appear more than once.
• A sliding window approach can efficiently extract all substrings of length 10 from the given string.
• A hash table (or dictionary) can be used to count occurrences of each substring.

Space and Time Complexity

Time Complexity: O(n), where n is the length of the input string. We scan through the string once and perform constant-time operations for each substring. Space Complexity: O(n), in the worst case, if all substrings are unique and stored in the hash table.

Solution

The solution involves using a sliding window approach to extract all possible 10-letter-long substrings from the given DNA sequence. We will utilize a hash table (or dictionary) to keep track of the count of each substring. If a substring appears more than once, we add it to the result list.

1. Initialize an empty hash table to count occurrences of each substring.
2. Use a loop to slide through the string and extract each 10-letter-long substring.
3. For each substring, update its count in the hash table.
4. After processing all substrings, filter the hash table to find which substrings have a count greater than 1, and return them as the result.

Code Solutions