187. Repeated DNA Sequences

https://leetcode.com/problems/repeated-dna-sequences/description/

All DNA is composed of a series of nucleotides abbreviated as A, C, G, and T, for example: "ACGAATTCCG". When studying DNA, it is sometimes useful to identify repeated sequences within the DNA.

Write a function to find all the 10-letter-long sequences (substrings) that occur more than once in a DNA molecule.

For example,

Given s = "AAAAACCCCCAAAAACCCCCCAAAAAGGGTTT",

Return:
["AAAAACCCCC", "CCCCCAAAAA"].

 

 

Sol:

 

public class Solution {
    public List<String> findRepeatedDnaSequences(String s) {
        
        // Brute Force. 
        // Time O(n) Space O(10*n)
        
        final List<String> result = new ArrayList<>();
        if (s.length() < 10) return result;
        
        final Map<String, Integer> counter = new HashMap<>();
        
        for (int i = 0; i < s.length() - 9; ++i){
            final String key = s.substring(i, i + 10);
            int value = counter.getOrDefault(key, 0);
            counter.put(key, value + 1);
            
        }
        
        for (Map.Entry <String, Integer> entry : counter.entrySet()){
            if (entry.getValue() > 1){
                result.add(entry.getKey());
            }
        }
        
        return result;
        
    }
}