<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 pre = "";
        std::string pre2 = "";
        int max = -1;
        bool flag = false;
        for(int i = 1; i <= str2.length(); ++i){
            pre = str2.substr(0,i);
            flag = false;
            if( str2.length() % i == 0 && str1.length() % i == 0){
                for(int j = 0; j < str1.length();){
                    pre2 = str1.substr(j,i);
                    j+=i;
                    //cout<<pre<<" "<<pre2<<" "<<j<<endl;
                    if(pre != pre2){
                        flag = false;
                        break;
                    }
                    if(j == str1.length()){
                        flag = true;
                    } 
                }
                for(int j = 0; flag == true && j < str2.length();){
                    pre2 = str2.substr(j,i);
                    j+=i;
                    //cout<<pre<<" "<<pre2<<" "<<j<<endl;
                    if(pre != pre2){
                        break;
                    }
                    if(j == str2.length() && flag){
                        max = i;
                    } 
                }
            }
        }
        return max == -1 ? "" : str2.substr(0,max);
    }
};
28009