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macro_rules! next_node_as_ref {
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  ($a: expr) => {
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    $a.as_ref().unwrap().next
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  };
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}
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macro_rules! next_node_as_mut {
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  ($a: expr) => {
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    $a.as_mut().unwrap().next
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  };
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}
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impl Solution {
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  pub fn swap_pairs(mut head: Option<Box<ListNode>>) -> Option<Box<ListNode>> {
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    if head.is_none() {
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      return None;
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    }
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    let mut cur_node = &mut head;
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    // while we have something to work with (.next has something)
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    while cur_node.is_some() && next_node_as_ref!(cur_node).is_some() {
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      // .take() Takes the value out of the option, leaving a None in its place.
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      let mut even_node = next_node_as_mut!(cur_node).take();
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      // taking next odd node's value
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      let next_odd_node = next_node_as_mut!(even_node).take();
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      // cur_node's .next points to next odd node
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      next_node_as_mut!(cur_node) = next_odd_node;
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      // even node's .next points to cur_node
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      next_node_as_mut!(even_node) = cur_node.take();
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      // switching cur_node with even node
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      cur_node.replace(even_node.unwrap());
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      // point cursor to .next.next
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      cur_node = &mut next_node_as_mut!(next_node_as_mut!(cur_node));
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    }
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    head
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  }
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}