如何删除最后放置的子节点?

我已经触摸开始发现,然后在用户点击屏幕时放置一个节点。

override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {

    if let touchLocation = touches.first?.location(in: sceneView)
    {
        let hitTestResults = sceneView.hitTest(touchLocation, types: .featurePoint)

        if let hitResult = hitTestResults.first
        {
            addDot(at: hitResult)
        }
    }
}

func addDot(at hitResult : ARHitTestResult)
{

    let imagePlane = SCNPlane(width: 0.1, height: 0.1)
    imagePlane.firstMaterial?.diffuse.contents = UIImage(named: "helmet_R.png")
 let planeNode = SCNNode(geometry: imagePlane)

   planeNode.position = SCNVector3(hitResult.worldTransform.columns.3.x, hitResult.worldTransform.columns.3.y, hitResult.worldTransform.columns.3.z)

    planeNode.constraints = [SCNBillboardConstraint()]


    sceneView.scene.rootNode.addChildNode(planeNode)

我尝试了一个简单的

planeNode.removeFromParentNode()