<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 repeat(const string& str,const string& mode)
    {
        auto size = mode.size();
        auto str_size = str.size();
        int i=size;
        int j=i+size;

        while(j<=str_size)
        {
            auto sub = str.substr(i,j-i);
            if(sub != mode)
            {
                return false;
            }
            i=j;
            j=i+size;
        }
        return i==str_size;
    }
    string gcdOfStrings(string str1, string str2) {
        if(str1==str2)
        {
            return str1;
        }
        if(str1.empty() || str2.empty())
        {
            return "";
        }

        auto n1 = str1.size();
        auto n2 = str2.size();

        int i = 0;
        for(i=0;i<n1 && i<n2;i++)
        {
            if(str1[i]!= str2[i])
            {
                break;
            }
        }

        i--;

        if(i<0)
        {
            return "";
        }

        for(int j=i;j>=0;--j)
        {
            auto mode=str1.substr(0,j+1);
            if(repeat(str1,mode) && repeat(str2,mode))
            {
                return mode;
            }
        }
        return "";
    }
};
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