除了pop方法之外，手动移除并释放viewcontroller

iOS

http://www.jianshu.com/p/b7fab12527e7

viewControllers.remove(at: thirdToLastIndex)
thirdToLastVC.willMove(toParentViewController: nil)
thirdToLastVC.view.removeFromSuperview()
thirdToLastVC.removeFromParentViewController()