關於popToViewController用法 (有點疑惑，求助攻！！！)

求小夥伴借個火   

popToViewController用法   


[self.navigationController popToViewController:[self.navigationController.viewControllers objectAtIndex:2] 
animated:YES];
或

for (UIViewController *temp in self.navigationController.viewControllers) {
           if ([temp isKindOfClass:[你要跳轉到的Controller class]]) {
              [self.navigationController popToViewController:temp animated:YES];
           }
       }

我理解從堆棧中取，只是爲了取同一個對象？假如，我是說假如，我把這個viewcontroller搞成一個單例的對象。爲什麼直接用系統的pop回不去呢，區別只是外層的遍歷判斷，還是單例？（求大神幫忙解惑！！！）


Printing description of view:
<OneViewController: 0x7fd134209020>
(lldb) po nag.viewControllers
<__NSSingleObjectArrayI 0x60000001c870>(
<OneViewController: 0x7fd134209020>
)


(lldb) po [OneViewController sharedInstance]
<OneViewController: 0x7fd134209020>

(lldb) po self.navigationController.viewControllers
<__NSArrayI 0x610000054f70>(
<OneViewController: 0x7fd134209020>,
<TwoViewController: 0x7fd1342122d0>,
<ThreeViewController: 0x7fd13240f130>
)


求小夥伴助攻，解個惑！！！感激不盡