<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) {
        int len1 = str1.length();
        int len2 = str2.length();

        // The length of the GCD is a CD of the lengths

        string candidate = "";
        
        for(int i=1; i<=min(len1, len2); i++) {
            if(len1%i != 0 || len2%i != 0) continue;
            if(str1.substr(0, i) != str2.substr(0, i)) return candidate;

            bool wrong = false;

            for(int j=0; j<max(len1, len2)/i; j++) {
                if(j<len1/i)
                    if(str1.substr(j*i, i) != str1.substr(0, i)) {
                        wrong = true;
                        break;
                    }
                        
                if(j<len2/i)
                    if(str2.substr(j*i, i) != str2.substr(0, i)) {
                        wrong = true;
                        break;
                    }
            }
            if(!wrong)
                candidate =  str2.substr(0, i);

        }
        return candidate;
    }
};
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