如何在后退按钮点击一次显示AdMob非页内广告

I have this code:

我有这个代码:

func createAndLoadInterstitial()  {
    interstitial = GADInterstitial(adUnitID: "ca-app-pub-xxxxxxxxxx/xxxxx")
    interstitial.delegate = self
    let request = GADRequest()
    interstitial.loadRequest(request)
}

override func viewDidAppear(animated: Bool) {
    if (interstitial.isReady && showAd) {
        showAd = false
        // print("iterstitialMain is ready")
        interstitial.presentFromRootViewController(self)
        self.createAndLoadInterstitial()
    }
    else {
        showAd = true
    }
}
func createAndLoa