Linked list frequency

impl Solution {
  pub fn frequencies_of_elements(head: Option<Box<ListNode>>) -> Option<Box<ListNode>> {
    let mut freq = std::collections::HashMap::new();
    let mut cur = &head;
    while let Some(node) = cur {
      *freq.entry(node.val).or_insert(0) += 1;
      cur = &node.next;
    }
    let mut result = None;
    for (_, &mut count) in freq.iter_mut() {
      let mut node = ListNode::new(count);
      node.next = result;
      result = Some(Box::new(node));
    }
    result
  }
}