Greatest Common Divisor of Strings LeetCode solution in Rust

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impl Solution {
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  pub fn gcd_of_strings(str1: String, str2: String) -> String {
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    if &(str1.clone() + &str2) != &(str2.clone() + &str1) {
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      return "".to_string();
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    }
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    let length = gcd(str1.len(), str2.len());
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    str1[0..length].to_owned()
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  }
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}
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pub fn gcd(n1: usize, n2: usize) -> usize {
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  if n2 == 0 {
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    return n1;
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  }
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  gcd(n2, n1 % n2)
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}