c++11 - how ! is different from != -

i writing function goal print list in pair of two, eg:

list : 1 2 3 4 5 6

pair list: 2 1 4 3 6 5

i wrote following code:

printpair(){     bool flag = 1;     node *temp = new node();     node *temp2 = new node();     temp2 = null;      if(!head)     {         printf("empty list!!");         return 0;     }     temp = head;      while(!temp && !temp->next)  //error here 1.     {         if(!temp2)  //error here 2.             temp2->next->next = temp->next;         temp2 = temp->next;         temp->next = temp->next->next;         temp2->next = temp;         if(flag)         {             head = temp2;             flag = 0;         }         temp = temp->next;     } } 

it works fine if replaced :

1. `while(temp !=null && temp->next !=null)` 2. `if(temp2 != null)` 

so how ! different != ?

while(!temp){} execute when temp has boolean value false.

while(temp != null) execute if temp has value other null (if temp true, pass)

you want use while(temp != null) because follow -> next references until reach last item in list (temp->next pointing nowhere)