HashtTable-187-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"].

Best Solution:

class Solution{//11ms
    public List findRepeatedDnaSequences(String s){
        List res = new ArrayList<>();
        if (s == null || s.length() < 10) {
            return res;
        }
        //对字母进行编码
        char[] map = new char[256];
        map['A'] = 0;
        map['C'] = 1;
        map['G'] = 2;
        map['T'] = 3;
        int mask = 0xfffff;//20bit,10个字母，每个字母占2bit
        int val = 0;
        char[] schar = s.toCharArray();
        for (int i = 0;i < 9 ;i ++ ) {//对前9位进行编码
            val = (val << 2) | (map[schar[i]] & 3);
        }
        byte[] bytes = new byte[1 << 20];
        for (int i = 9;i < schar.length ;i ++ ) {
        //注意这里与mask&,超出部分截取
            val = ((val << 2) & mask) | ((map[schar[i]]) & 3);//编码
            if (bytes[val] == 1) {
                res.add(String.valueOf(schar,i - 9,10));
            }
            //这里很关键,字节大小有限，若超过限制变为1，会重复加
            if (bytes[val] < 2)   bytes[val]++;
        }
        return res;
    }
}