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impl Solution {
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  pub fn partition(mut head: Option<Box<ListNode>>, x: i32) -> Option<Box<ListNode>> {
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    let mut dummy1: Option<Box<ListNode>> = None;
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    let mut dummy2: Option<Box<ListNode>> = None;
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    let mut r1 = &mut dummy1;
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    let mut r2 = &mut dummy2;
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    while head.is_some() {
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      if head.as_ref().unwrap().val < x {
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        if r1.is_some() {
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          r1 = &mut r1.as_mut().unwrap().next;
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        }
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        std::mem::swap(r1, &mut head);
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        std::mem::swap(&mut r1.as_mut().unwrap().next, &mut head);
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      } else {
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        std::mem::swap(r2, &mut head);
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        std::mem::swap(&mut r2.as_mut().unwrap().next, &mut head);
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        r2 = &mut r2.as_mut().unwrap().next;
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      }
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    }
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    if r1.is_some() {
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      r1.as_mut().unwrap().next = dummy2;
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      dummy1
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    } else {
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      dummy2
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    }
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  }
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}