[leedcode 187] Repeated DNA Sequences

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"].

public class Solution {
    public List<String> findRepeatedDnaSequences(String s) {
        //因为只有4个字母,所以可以创建自己的hashkey, 每两个BITS, 对应一个 incoming character. 超过20bit 即10个字符时, 只保留20bits.
        Map<Character,Integer> map=new HashMap<Character,Integer>();
        map.put('A',0);
        map.put('C',1);
        map.put('G',2);
        map.put('T',3);
        List<String> res=new ArrayList<String>();
        int hash=0;
        Set<Integer> set=new HashSet<Integer>();
        for(int i=0;i<s.length();i++){
            char c=s.charAt(i);
            if(i<9){
                hash=(hash<<2)+map.get(c);
            }else{
                hash=(hash<<2)+map.get(c);
                hash&=(1<<20)-1;
                if(set.contains(hash)){
                    if(!res.contains(s.substring(i-9,i+1)))
                    res.add(s.substring(i-9,i+1));
                }else{
                    set.add(hash);
                }
            }
        }
        return res;
    }
}