[LeetCode]Repeated DNA Sequences

Question
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”].

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本题难度Medium。

集合法

【复杂度】
时间 O(N) 空间 O(N)

【思路】
利用集合set存放遍历过1次的字符串，利用集合ans存放遍历过2次的字符串。

【注意】
这里ans使用集合而不用List的原因就在于去除重复。

【代码】

public class Solution {
    public List<String> findRepeatedDnaSequences(String s) {
        //require
        Set<String> ans=new HashSet<>();
        if(s==null||s.length()<10)return new LinkedList<String>(ans);
        Set<String> set=new HashSet<>();
        //invariant
        for(int i=0;i+10<=s.length();i++){
            String str=s.substring(i,i+10);
            if(set.contains(str))ans.add(str);
            else                 set.add(str);
        }
        //ensure
        return new LinkedList<String>(ans);
    }
}