iOS 深入理解UINavigationController 和 UIViewController 之间的关系
创建三个类

BasicViewController : UIViewController SecondViewController : UIViewController ThirdViewController : UIViewController 

然后我们在BasicViewController .m文件中push一个viewController：
SecondViewController *svc = [SecondViewController new];
[self.navigationController pushViewController:svc animated:true];
在SecondViewController.m文件中pop出viewController：
[self.navigationController popViewControllerAnimated:true]

问题就来了，
push和pop是同一个viewController，那为什么用self.navigationController 就可以知道了

其中在AppDelegate.m：

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions{    self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];    // Override point for customization after application launch.    BasicViewController  *basicViewController = [BasicViewController new];//实例化内存后，_parentViewController-->nil    UINavigationController *navigationController = [[UINavigationController alloc] initWithRootViewController:basicViewController];    // self.window.rootViewController = basicViewController.parentViewController;     self.window.rootViewController = navigationController;        [self.window makeKeyAndVisible];    return YES;}

等价于

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions{    self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];    // Override point for customization after application launch.    BasicViewController  *basicViewController = [BasicViewController new];//实例化内存后，_parentViewController-->nil    UINavigationController *navigationController = [[UINavigationController alloc] initWithRootViewController:basicViewController];    self.window.rootViewController = basicViewController.parentViewController; //不可以注释上一句，因为上一句是为_parentViewController实例化    // self.window.rootViewController = navigationController;        [self.window makeKeyAndVisible];    return YES;}

分配内存如图