leetCode练习（187）

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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,

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<String> findRepeatedDnaSequences(String s) {HashSet<String> first = new HashSet<>();HashSet<String> second = new HashSet<>();List<String> list = new ArrayList<>();for(int i=0;i<s.length()-9;i++){String str = s.substring(i,i+10);if(first.contains(str)){second.add(str);}else{first.add(str);}}for(String w:second){list.add(w);}return list;    }}

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