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"].


java的int占了4个字节位，总共32位；本题是将A，T，C，G四个字母每个字母占两位进行考虑，十个字母长度就是20位，每达到20位就存找到hashset里面。答案有一个地方非常巧妙，就是创建
两个hashset，一个来存储出现一次，另一个hashset来存储出现第二次，这里面有一个细节，在if条件句里面&&连接的两个hashset，如果第一个为false，那么第二个将不会执行；代码如下：

public class Solution {
    public List<String> findRepeatedDnaSequences(String s) {
        List<String> res = new ArrayList<String>();
        Set<Integer> words = new HashSet<Integer>();
        Set<Integer> doublewords = new HashSet<Integer>();
        int[] map = new int[26];
        map[0] = 0;
        map['C'-'A'] = 1;
        map['T'-'A'] = 2;
        map['G'-'A'] = 3;
        for(int i=0;i<s.length()-9;i++){
            int v = 0;
            for(int j=i;j<i+10;j++){
                v<<=2;
                v|=map[s.charAt(j)-'A'];
            }
            if(!words.add(v)&&doublewords.add(v)){
                res.add(s.substring(i,i+10));
            }
        }
        return res;
    }
}