<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>

#include <sstream>
#include <iostream>

class Solution {
public:
    string gcdOfStrings(string str1, string str2) {
        if(str1.empty() || str2.empty()){
            return "";
        }

        string divisor = str1;

        if(str2.size() < str1.size()){
            divisor = str2;
        }

        //Loop though shorter string and see if its a divisor and if it's not, remove a character
        for(int i = divisor.length(); i >= 0 ; i--){
            divisor = divisor.substr(0, i);
            if(isDivisor(str2,divisor) && isDivisor(str1, divisor)){
                return divisor;
            }
        }

        return ""; 
    }

    //Check if substring is divsor 
    bool isDivisor(string str, string divisor){
        std::stringstream result;
        if(str.size() == 0 || divisor.size() == 0 || str.size() < divisor.size() || str.size() % divisor.size() != 0){
            return false;
        }

        int numRepeat = str.size() / divisor.size();

        for(int i = 0; i < numRepeat; i++){
            result << divisor; 
        }

        return str == result.str();
    }

};
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