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"].

比较经典的题目
思路：用hashmap扫一遍，出现过的就加到list上去，为了不使结果重复，当碰到重复的时候更新value。

public class Solution {
    public List<String> findRepeatedDnaSequences(String s) {
        List<String> res=new ArrayList<>();
        if(s==null)return res;
        Map<String,Integer> check=new HashMap<>();
        for(int i=0;i<=s.length()-10;i++){
            String sub=s.substring(i,i+10);
            if(!check.containsKey(sub)){
                check.put(sub,1);
            }else{
                if(check.get(sub)==1){
                    res.add(sub);
                    check.put(sub,2);
                }
            }
        }
        return res;
    }
}