// Definition for singly-linked list.
#[derive(PartialEq, Eq, Clone, Debug)]
pub struct ListNode {
  pub val: i32,
  pub next: Option<Box<ListNode>>
}

impl ListNode {
  #[inline]
  fn new(val: i32) -> Self {
    ListNode {
      next: None,
      val
    }
  }
}

struct Solution {}

impl Solution {
    pub fn reverse_list(head: Option<Box<ListNode>>) -> Option<Box<ListNode>> {
        let mut prev = None;
        let mut cur = head;
        let mut next = None;
        while cur.is_some() {
            let mut n = cur.take().unwrap();
            next = n.next.take();
            n.next = prev;
            cur = next;
            prev = Some(n);
        }
        prev
    }
}