In order leetcode 94. [binary tree traversal time defeating 99.19% 39.48%] [memory] beat

 

It turns out al class members as recycling does not have any optimization

 

public List<Integer> inorderTraversal(TreeNode root) {
        ArrayList<Integer>al=new ArrayList<>();
        if (root==null)return al;
        al.addAll(inorderTraversal(root.left));
        al.add(root.val);
        al.addAll(inorderTraversal(root.right));
        return al;
    }