LeetCode.187 Repeated DNA Sequences

题目：

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"].

分析1（推荐）：

class Solution {
    public List<String> findRepeatedDnaSequences(String s) {
        //给定字符串，返回由其中10个字符组成的重复
        //思路：使用两个HashSet实现
        List<String> list=new ArrayList<>();
        if(s.length()<10)return list;
        
        Set<String> set1=new HashSet<>(),set2=new HashSet<>();
        for(int i=0;i<s.length()-9;i++){
            String str=s.substring(i,i+10);
            if(!set1.add(str)){
                //没有添加成功说明存在相同的,用set2来存储，类似两级存储，而set2不会重复存储元素
                set2.add(str);
            }
        }
        list=new ArrayList(set2);
        return list;
    }
}

分析2（易理解）：

class Solution {
    public List<String> findRepeatedDnaSequences(String s) {
        //给定字符串，返回由其中10个字符组成的重复
        List<String> list=new ArrayList<>();
        if(s.length()<10){
            return list;
        }
        HashMap<String,Integer> hm=new HashMap<String,Integer>();
        
        for(int i=0;i<s.length()-9;i++){
            String str=s.substring(i,i+10);
            if(hm.containsKey(str)){
                int newValue=hm.get(str)+1;
                hm.put(str,newValue);
            }else{
                hm.put(str,1);
            }
        }
        
        //遍历HashMap(方式1)
        //Map.Entry(K,V)是Map下的一个接口
        // for(Map.Entry<String,Integer> entry:hm.entrySet()){
        //     if(entry.getValue()>1){
        //         list.add(entry.getKey());
        //     }
        // }
        
         //遍历HashMap(方式2)
        for(String key:hm.keySet()){
            if(hm.get(key)>1){
                list.add(key);
            }
        }
        
        return list;
    }
}