leetCode练习（187）

题目：Repeated DNA Sequences

难度：MEDIUM

问题描述：

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,

For example,

Given s = "AAAAACCCCCAAAAACCCCCCAAAAAGGGTTT",

Return:
["AAAAACCCCC", "CCCCCAAAAA"].

解题思路：使用一个Set1保存所有未曾出现过的str，使用另一个Set2保存已经出现在Set1中的str。

代码如下：

public class Solution {
    public List findRepeatedDnaSequences(String s) {
		HashSet first = new HashSet<>();
		HashSet second = new HashSet<>();
		List list = new ArrayList<>();
		for(int i=0;i