描写叙述：

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"].

思路：

1.非常显然，暴力求解也是一种方法。虽然该方法是不可能的。

2.我们首先来看字母 ”A" "C" “G" "T" 的ASCII码，各自是65, 67, 71, 84，二进制表示为 1000001, 1000011, 1000111, 1010100。能够看到它们的后三位是不同，所以用后三位就能够区分这四个字母。一个字母用3bit来区分，那么10个字母用30bit就够了。用int的第29~0位分表表示这0~9个字符，然后把30bit转化为int作为这个子串的key，放入到HashTable中。以推断该子串是否出现过。

代码：

public List
      
        findRepeatedDnaSequences(String s)	{		List
       
        list=new ArrayList
        
         ();		int strLen=s.length();		if(strLen<=10)			return list;		HashMap
         
          map=new HashMap
          
           ();		int key=0;		for(int i=0;i