<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) {
        std::string smaller_str;
        if (str1.size() <= str2.size()) {
            smaller_str = str1;
        }
        else {
            smaller_str = str2;
        }
        
        for (int i=smaller_str.size()-1;i>=0;i--) {
            if(checkSubstr(str1, smaller_str, i) && checkSubstr(str2, smaller_str, i)) {
                return smaller_str.substr(0, i+1);
            }
        }
        
        return "";
    }
    
    bool checkSubstr(const std::string& target_str, const std::string& smaller_str, int i) {
        std::string target_sub = smaller_str.substr(0, i+1);
        int multiplier = target_str.size() / target_sub.size();
        
        std::string tmp_str;
        tmp_str.reserve(target_str.size());
        for(int i=0;i<multiplier;i++) {
            tmp_str+=target_sub;
        }
        
        return tmp_str == target_str;
    }
};
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