Preorder Traversal without Recursion

PreorderIterative.java

 public ArrayList<Integer> preorderTraversal(TreeNode A) {
        
        if(A == null) return null;
        Stack<TreeNode> stack = new Stack<TreeNode>();
        ArrayList<Integer> result = new ArrayList<Integer>();
        stack.push(A);
        
        while(!stack.isEmpty()){
            TreeNode temp = stack.pop();
            result.add(temp.val);
            if(temp.right != null){
                stack.push(temp.right);
            } 
            if(temp.left != null){
                stack.push(temp.left);
            }
        }
        return result;
    }