leetcode(187):Repeated DNA Sequences

原題：
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：題意爲給定一個字符串，要求出現次數大於1的長度爲10的子串集合。

思路是遍歷一遍字符串，截取長度爲10的子串，記錄出現的次數，很容易想到map

Java實現如下：

public class Solution {
    public List<String> findRepeatedDnaSequences(String s) {
        List<String> ls = new ArrayList<String>();
        if(s.length()==0 || s==null) return ls;
        int len = s.length();
        Map<String,Integer> map = new HashMap<String,Integer>();
        for (int i = 0; i <= len-10; i++) {
            String str = s.substring(i,i+10);
            if(map.containsKey(str)){
                map.put(str, map.get(str)+1);//重複出現的子串次數+1
            }else{
                map.put(str, 1);//新出現的子串加入map
            }
        }
        for(Map.Entry<String, Integer> ent:map.entrySet()){
            if(ent.getValue()>1) ls.add(ent.getKey());
        }
        return ls;
    }
}