reference counted tree pruning example

gistfile1.txt

use std::{rc::Rc, cell::RefCell};
#[derive(Debug)]
struct TreeNode {
    val: i32,
    left: Option<Rc<RefCell<TreeNode>>>,
    right: Option<Rc<RefCell<TreeNode>>>
}

impl Drop for TreeNode { fn drop(&mut self) { println!(">>> Free Node {}", self.val); } }

fn node(val: i32, left: Option<Rc<RefCell<TreeNode>>>, right: Option<Rc<RefCell<TreeNode>>>) -> Option<Rc<RefCell<TreeNode>>> {
    Some(Rc::new(RefCell::new(TreeNode { val, left, right })))
}

fn main() {
    let (n4, n5, n6, n7) = (node(4, None, None), node(5, None, None), node(6, None, None), node(7, None, None));
    let (n2, n3)         = (node(2, n4, n5), node(3, n6, n7));
    let root             = node(1, n2, n3);

    let keep = root.as_ref().unwrap().borrow().left.clone();
    println!("count on n2: {}", Rc::strong_count(&keep.as_ref().unwrap()));

    println!("dropping root");
    drop(root);
    println!("count on n2: {}", Rc::strong_count(&keep.as_ref().unwrap()));

    println!("the left tree value are also usable {} {}", 
        keep.as_ref().unwrap().borrow().left.as_ref().unwrap().borrow().val,
        keep.as_ref().unwrap().borrow().right.as_ref().unwrap().borrow().val
    );
    println!("exiting the main");
}