Greatest Common Divisor of Strings LeetCode solution in C++

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class Solution {
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 public:
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  string gcdOfStrings(string str1, string str2) {
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    int m = str1.length(), n = str2.length();
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    int temp = __gcd(m, n);
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    string str;
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    if (str1 + str2 == str2 + str1) str = str1.substr(0, temp);
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    return str;
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  }
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};