Please explain the codes in each line. queue lab, c lang /* 

 Return number of elements in queue. 

 Return 0 if q is NULL or empty 

 */ 

int q_size(queue_t *q) 

{ 

 /* You need to write the code for this function */ 

 /* Remember: It should operate in O(1) time */ 

if(q != NULL) // ??? 

 return q->n; // ???? 

 return 0; // ??? 

}