leetcode repetitive DNA sequences

All DNA by a series of abbreviated as A, C, G, and T nucleotides, for example: "ACGAATTCCG". In the study of DNA, DNA repeat sequence recognition in the research will be very helpful sometimes.

A function to find the write occurs over a plurality of long sequence of 10 letters (substring) all of the DNA molecule.

Example:

Input: s = "AAAAACCCCCAAAAACCCCCCAAAAAGGGTTT"

输出: ["AAAAACCCCC", "CCCCCAAAAA"]

Source: stay button (LeetCode)
link: https://leetcode-cn.com/problems/repeated-dna-sequences
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A algorithm: violence enumeration
algorithm II: hash table optimization cycle of the second dimension, the values are kept up, you can find

class Solution {
public:
    vector<string> findRepeatedDnaSequences(string s) {
        unordered_map<string,int> hash;
        vector<string> res;
        for(int i=0;i+10<=s.size();++i){
            string now = s.substr(i,10);
            if(hash[now]==1)res.push_back(now);
            hash[now]++;
        }
        return res;
    }
};