// The view controller that presented this view controller (or its farthest ancestor.)
@property(nonatomic,readonly) UIViewController *presentingViewController NS_AVAILABLE_IOS(5_0);
a present b，b present c 。 现在要从c中直接回到a
if ([self respondsToSelector:@selector(presentingViewController)])
{
[self.presentingViewController dismissViewControllerAnimated:YES completion:NULL];
{
[self.presentingViewController dismissViewControllerAnimated:YES completion:NULL];
}