Delete Nodes and Return Forest

def delNodes(root: TreeNode, to_delete: List[int]) -> List[TreeNode]:
    delete = set(to_delete)
    forest = []

    def _prune(node, isRoot):
        if node is None:
            return
        if node.val in delete:
            _prune(node.left, True)
            _prune(node.right, True)
        else:
            if isRoot:
                forest.append(node)
            _prune(node.left, False)
            _prune(node.right, False)

            if node.left and node.left.val in delete:
                node.left = None

            if node.right and node.right.val in delete:
                node.right = None

    _prune(root, True)

    return forest