how ! is different from !=

I was writing function for the following : /* print list in pair of two : eg: list : 1 2 3 4 5 6 pair list: 2 1 4 3 6 5*/.

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 with : 1.while(temp !=NULL && temp->next !=NULL) 2. if(temp2 != NULL) */ So how is this ! is different from !=