Find Largest Value in Each Tree Row LeetCode solution in Rust

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use std::cell::RefCell;
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use std::cmp::max;
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use std::rc::Rc;
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impl Solution {
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  pub fn largest_values(root: Option<Rc<RefCell<TreeNode>>>) -> Vec<i32> {
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    let mut res: Vec<i32> = vec![];
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    let mut queue = vec![root];
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    while !queue.is_empty() {
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      let k = queue.len();
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      let mut current_max = i32::MIN;
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      for _i in 0..k {
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        match queue.remove(0) {
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          Some(r) => {
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            current_max = max(current_max, r.borrow().val);
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            if !r.borrow().left.is_none() {
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              queue.push(r.borrow().left.clone());
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            }
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            if !r.borrow().right.is_none() {
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              queue.push(r.borrow().right.clone());
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            }
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          }
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          None => return res,
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        };
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      }
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      res.push(current_max);
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    }
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    return res;
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  }
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}