1, A->B :从A present到B 直接在B控制器中执行self.dismiss 就能回到A
2.A->B->C :A present到B 再从B present到C 在C中执行 self.presentingViewController?.presentingViewController?.dismiss(animated: true, completion:nil) 回到A
3A->B->C->.......N 直接通过 window.rootViewController 获取根VC,再dissmiss
self.view.window?.rootViewController?.dismiss(animated: true, completion: nil)