convert vector of i32s into a LeetCode linked list

vec_to_list.rs

impl Solution {
    // ...stuff...
    
    // Returns a LeetCode linked list from the given vec.
    fn vec_to_list(vec: Vec<i32>) -> Option<Box<ListNode>> {
        let mut root: Option<Box<ListNode>> = None;
        let mut node = &mut root;
        for v in vec {
            node.replace(Box::new(ListNode::new(v)));
            if let Some(n) = node {
                node = &mut n.next;
            }
        }
        root
    }
    
    // ...more stuff...
}