Repeated DNA Sequences

比较难的一道题，但是还是能看懂

public class Solution {

    public List<String> findRepeatedDnaSequences(String s) {

        // ref http://blog.csdn.net/haiyi727/article/details/43752693

        int len = s.length();

        HashSet<Integer> tmp = new HashSet<Integer>();

        HashSet<Integer> resSet = new HashSet<Integer>();

        ArrayList<String> res = new ArrayList<String>();

        if(len<10) return res;

        HashMap<Character, Integer> map = new HashMap<Character, Integer>();

        map.put('A',0);

        map.put('T',1);

        map.put('C',2);

        map.put('G',3);

        int hash = 0;

        for(int i=0;i<len;i++){

            if(i<9){

                hash = (hash<<2) + map.get(s.charAt(i));

            }else{

                hash = (hash<<2)+ map.get(s.charAt(i));

                hash &= (1<<20)-1; // 取the last 20 digits

                if(tmp.contains(hash)&& !resSet.contains(hash)){

                    resSet.add(hash);

                    res.add(s.substring(i-9,i+1));

                }else{

                    tmp.add(hash);

                }

            }

        }

        return res;

    }

}