inOrder_traversal

private void iterativeInOrder(Node node){
        // we are going to the left most item
        if (node == null) return;
        // create an empty stack and push root to it.
        Stack<Node> stack = new Stack<>();

        while (node != null || !stack.isEmpty()){
            // 1. inOrder(left_child)   // put the data into stack
            // 2. Print(node.data)
            // 3. inOrder(right_child)
            if (node != null){
                stack.push(node);
                node = node.left;
            }else{
                node = stack.pop();
                System.out.print(node.data + " ");
                node = node.right;
            }
        }
    }