<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 {
private:
    
public:
    vector<string> findRepeatedDnaSequences(string s) {
        unordered_set<int> seen;
        unordered_set<int> dup;
        vector<string> result;
        vector<char> m(26);
        m['A' - 'A'] = 0;
        m['C' - 'A'] = 1;
        m['G' - 'A'] = 2;
        m['T' - 'A'] = 3;
        
        for (int i = 0; i + 10 <= s.size(); ++i) {
            string substr = s.substr(i, 10);
            int v = 0;
            for (int j = i; j < i + 10; ++j) { //20 bits < 32 bit int
                v <<= 2;
                v |= m[s[j] - 'A'];
            }
            if (seen.count(v) == 0) { //not seen
                seen.insert(v);
            } else if (dup.count(v) == 0) { //seen but not dup
                dup.insert(v);
                result.push_back(substr);
            } //dup
        }
        return result;
    }
};
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