Diagonal Traversal of Binary Tree

/* Node is defined as
class Node{
    int data;
    Node left;
    Node right;
    Node(int data){
        this.data = data;
        left=null;
        right=null;
    }
}*/

class Tree {
    public ArrayList<Integer> diagonal(Node root) {
        ArrayList<Integer> res = new ArrayList<> ();
        diagonalUtil(root, res);
        return res;
    }
    
    private void diagonalUtil(Node root, ArrayList<Integer> res) {
        if(root == null) 
            return;
        
        Queue<Node> que = new LinkedList<> ();
        
        while(root != null) {
            res.add(root.data);
            if(root.left != null) {
                que.add(root.left);
            }
            if(root.right != null) {
                root = root.right;
            }
            else {
                if(que.isEmpty() == false) {
                    root = que.remove();
                }
                else {
                    root = null;
                }
            }
        }
        
    }
}