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.

public class Solution {
    public List<String> findRepeatedDnaSequences(String s) {
        List<String> result = new ArrayList<>();
        if (s == null || s.length() == 0) return result;

        HashMap<String, Integer> map = new HashMap<>();
        for (int i = 0; i + 9 < s.length(); i++) {
            String newS = s.substring(i, i + 10);
            if (map.containsKey(newS) && map.get(newS) <= 1) {
                result.add(newS);
            }
            map.put(newS, map.getOrDefault(newS, 0) + 1);
        }
        return result;
    }
}

class Solution(object):
    def findRepeatedDnaSequences(self, s):
        """
        :type s: str
        :rtype: List[str]
        """
        if not s:
            return []

        dna_map = {}
        ret = []
        for i in xrange(len(s) - 9):
            dna = s[i : i + 10]
            if dna_map.get(dna, 0) == 1:
                ret.append(dna)
            dna_map[dna] = dna_map.get(dna, 0) + 1
        return ret