The correct answer will be 2, 1, 4, 3, 6, 5, 7.
if (!list || !list -> next) return // This line will check if the list is NULL, or having only one element.
q = p? p-> next : 0 // This line will assign the value of q. if p is not null, then next of it will be assigned to q otherwise the value of q will be 0. If we will not write this line, we may get a segmentation fault.