pre_order.rs

use std::rc::Rc;
use std::cell::RefCell;
impl Solution {
    pub fn preorder_traversal(root: Option<Rc<RefCell<TreeNode>>>) -> Vec<i32> {
        let mut result = vec![];
        
        fn helper(root: Option<Rc<RefCell<TreeNode>>>, result: &mut Vec<i32>) -> (){
            if let Some(curr) = root{
                
                let curr = curr.borrow();
                result.push(curr.val);
                
                helper(curr.left.clone(),result);
                helper(curr.right.clone(),result);
                
            }
        }
        
        helper(root, &mut result);
        
        return result;
    }
}