LeetCode 187: Repeated DNA Sequences

Explanation

To solve this problem, we can use a sliding window of size 10 to iterate through the input DNA sequence. We will keep track of all the 10-letter-long substrings we have seen so far using a HashMap to store the substrings and their frequencies. If we encounter a substring that has already been seen, we add it to the result list.

1. Create a HashMap to store the substrings and their frequencies.
2. Iterate through the input DNA sequence using a sliding window of size 10.
3. For each window, check if the substring is already in the HashMap.
4. If the substring is not in the HashMap, add it with a frequency of 1.
5. If the substring is already in the HashMap, increment its frequency and check if it has occurred more than once.
6. Add the repeated substrings to the result list.
7. Return the list of repeated substrings.

Time Complexity: O(n), where n is the length of the input DNA sequence. Space Complexity: O(n), where n is the number of unique 10-letter-long substrings in the input DNA sequence.

import java.util.*;

class Solution {
    public List<String> findRepeatedDnaSequences(String s) {
        List<String> result = new ArrayList<>();
        Map<String, Integer> substrings = new HashMap<>();
        
        for (int i = 0; i <= s.length() - 10; i++) {
            String sub = s.substring(i, i + 10);
            substrings.put(sub, substrings.getOrDefault(sub, 0) + 1);
            
            if (substrings.get(sub) == 2) {
                result.add(sub);
            }
        }
        
        return result;
    }
}