Swift2で NavigationController の子ウィンドウ閉じるとき2画面前に遷移させる

ThirdViewController.swift

class ThirdViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        
        // 1つ前のViewControllerを遷移リストから削除
        let parentIndex = (self.navigationController?.viewControllers.indexOf(self))! - 1
        self.navigationController?.viewControllers.removeAtIndex(parentIndex)
    }