<p>The <strong>DNA sequence</strong> is composed of a series of nucleotides abbreviated as <code>&#39;A&#39;</code>, <code>&#39;C&#39;</code>, <code>&#39;G&#39;</code>, and <code>&#39;T&#39;</code>.</p>

<ul>
	<li>For example, <code>&quot;ACGAATTCCG&quot;</code> is a <strong>DNA sequence</strong>.</li>
</ul>

<p>When studying <strong>DNA</strong>, it is useful to identify repeated sequences within the DNA.</p>

<p>Given a string <code>s</code> that represents a <strong>DNA sequence</strong>, return all the <strong><code>10</code>-letter-long</strong> sequences (substrings) that occur more than once in a DNA molecule. You may return the answer in <strong>any order</strong>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<pre><strong>Input:</strong> s = "AAAAACCCCCAAAAACCCCCCAAAAAGGGTTT"
<strong>Output:</strong> ["AAAAACCCCC","CCCCCAAAAA"]
</pre><p><strong class="example">Example 2:</strong></p>
<pre><strong>Input:</strong> s = "AAAAAAAAAAAAA"
<strong>Output:</strong> ["AAAAAAAAAA"]
</pre>
<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= s.length &lt;= 10<sup>5</sup></code></li>
	<li><code>s[i]</code> is either <code>&#39;A&#39;</code>, <code>&#39;C&#39;</code>, <code>&#39;G&#39;</code>, or <code>&#39;T&#39;</code>.</li>
</ul>

class Solution {
public:
    vector<string> findRepeatedDnaSequences(string s) {
        unordered_set<string> output;
        int L = 10;
        unordered_set<int> seen;
        if(s.length()<=10) return {};
        unordered_map<char,int> to_int = {{'A',1},{'C',2},{'G',3},{'T',4}};
        vector<int> nums(s.length());
        for(int i=0;i<s.length();i++) nums[i] = to_int[s[i]];
        int hash = 0;
        int base = 4;
        int aL = pow(base, L);
        for(int i=0;i<=s.length()-L;i++){
            if(i!=0){
                hash = hash*base - nums[i-1]*aL + nums[i+L-1];
            }
            else {
                for (int j = 0; j < L; j++) hash = hash * base + nums[j];
            }
            if (seen.find(hash) != seen.end()) output.insert(s.substr(i, L));
            seen.insert(hash);
        }
        return vector<string>(output.begin(), output.end());
        
    }
};
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