leetcode Repeated DNA Sequence

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"].

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这道题思路是这样的：ACGT的ASCII码值后三位是不同的，分别为001 011 111 100 也就是说通过3bit可以表示一个基因，30bit可以表示一个10-letter-long的基因组，我们只需要将这个30bit的整数存入hashmap，然后每次判断新的整数是否存在于map中即可：

public List<String> findRepeatedDnaSequences(String s) {    List<String> list=new ArrayList<>();    Map<Integer,Integer> map=new HashMap<>();    int gene=0;    for(int i=0;i<s.length();i++){        gene=(gene<<3|(s.charAt(i)&0x7))&(0x3fffffff);        if(i<9) continue;        if(map.get(gene)==null){            map.put(gene,1);        }        else if(map.get(gene)==1){            list.add(s.substring(i-9,i+1));            map.put(gene,2);        }    }    return list;}