<p>For two strings <code>s</code> and <code>t</code>, we say &quot;<code>t</code> divides <code>s</code>&quot; if and only if <code>s = t + t + t + ... + t + t</code> (i.e., <code>t</code> is concatenated with itself one or more times).</p>

<p>Given two strings <code>str1</code> and <code>str2</code>, return <em>the largest string </em><code>x</code><em> such that </em><code>x</code><em> divides both </em><code>str1</code><em> and </em><code>str2</code>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> str1 = &quot;ABCABC&quot;, str2 = &quot;ABC&quot;
<strong>Output:</strong> &quot;ABC&quot;
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> str1 = &quot;ABABAB&quot;, str2 = &quot;ABAB&quot;
<strong>Output:</strong> &quot;AB&quot;
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> str1 = &quot;LEET&quot;, str2 = &quot;CODE&quot;
<strong>Output:</strong> &quot;&quot;
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= str1.length, str2.length &lt;= 1000</code></li>
	<li><code>str1</code> and <code>str2</code> consist of English uppercase letters.</li>
</ul>

class Solution {
  private:
    inline string getCommonPrefix(const string &str1,
                                  const string &str2) const {
        size_t i = 0;
        for (; i < str1.size() and i < str2.size(); ++i) {
            if (str1[i] != str2[i])
                break;
        }
        return str1.substr(0, i);
    }

    inline bool isDivisible(const string &dividend,
                            const string &divisor) const {
        if (divisor.size() > dividend.size())
            return false;

        for (size_t i = 0; i < dividend.size(); i += divisor.size()) {
            if (dividend.substr(i, divisor.size()) != divisor)
                return false;
        }

        return true;
    }

  public:
    string gcdOfStrings(string str1, string str2) {
        string res = getCommonPrefix(str1, str2);

        while (!res.empty()) {
            if (isDivisible(str1, res) and isDivisible(str2, res))
                return res;
            res.pop_back();
        }

        return res;
    }
};
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