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impl Solution {
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  pub fn thousand_separator(n: i32) -> String {
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    n.to_string()
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      .chars()
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      .collect::<Vec<_>>()
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      .rchunks(3)
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      .rev()
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      .map(|v| v.iter().collect::<String>())
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      .collect::<Vec<_>>()
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      .join(".")
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  }
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}