<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 {
public:
    bool checkDivisibility(string s1, string s2) {
        if (s1.size() % s2.size() != 0)
            return false;

        int idx = 0, len = s2.size(), loop = s1.size() / s2.size();
        while (loop--) {
            if (s1.substr(idx, len) != s2) {
                return false;
            }
            idx += len;
        }
        return true;
    }
    string gcdOfStrings(string str1, string str2) {
        if (str1.size() < str2.size()) {
            string temp = str1;
            str1 = str2;
            str2 = temp;
        }

        if (checkDivisibility(str1, str2))
            return str2;

        for (int idx = str2.size() / 2; idx > 0; idx--) {
            string divisor = str2.substr(0, idx);
            if (checkDivisibility(str2, divisor) &&
                checkDivisibility(str1, divisor))
                return divisor;
        }

        return "";
    }
};
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