<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:
    string gcdOfStrings(string str1, string str2) {
        string gcd = "";
        int gcdLen = 0;
        if (str1.length() == str2.length()) {
            if (str1 == str2) return str1;
            else return "";
        }

        if (str1.length() < str2.length()) {
            std::swap(str1, str2);
        }

        for (int i = 1; i <= str2.length(); ++i) {
            string s = str2.substr(0, i);
            if (isStrDivisor(str2, s)) {
                if (isStrDivisor(str1, s)) {
                    if (i > gcdLen) {
                        gcd = s;
                        gcdLen = i;
                    }
                }
            }
        }

        return gcd;
    }

    bool isStrDivisor(string s, string div) {
        for (int i = 0; i < s.length(); i += div.length()) {
            if (i > s.length() - div.length())
                return false;
            if (s.substr(i, div.length()) != div)
                return false;
        }
        return true;
    }
};
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