struct Node { key:i32, left:Option<Box<Node>>, right:Option<Box<Node>> }
fn sample()->Node { Node {key:5,left:Some(Box::new(Node {key:3,left:None,right:None})),right:Some(Box::new(Node {key:7,left:None,right:None}))} }

fn show(n: &Node, depth: usize) {
    {
        let mut line = "  ".repeat(depth);
        line.push_str(&n.key.to_string());
        println!("{line}");
    }
    if let Some(left) = &n.left { show(left, depth + 1); }
    if let Some(right) = &n.right { show(right, depth + 1); }
}

fn main() {show(&sample(),0);}