<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 valid(string s1,string s2,int k)
     {
        int l1 = s1.length();
        int l2 = s2.length();
        if(l1%k > 0 || l1%k >0)
        return false;
        else
        {
            string base=s1.substr(0,k);
            int n1 = l1/k ,n2=l2/k;
            return (s1== joinwords(base,n1) && s2==joinwords(base,n2));

        }

     }
     string joinwords(string str, int k)
     {
        string ans = "";
        for(int i=0;i<k;++i)
        {
            ans += str;
        }
        return ans;
     }
    string gcdOfStrings(string str1, string str2) {
        int len1 = str1.length();
        int len2 =str2.length();
        for(int i = min(len1,len2);i>=1;--i)
        {
            if(valid(str1,str2,i))
                return str1.substr(0,i);
        }
        return "";
    }
};
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