Question

This is a recursive function to delete all even nodes from a linked list.

node *deleteEven(node *head)

if (head == NULL)

   return head;

  

if (head->data % 2 == 0)

{

node *temp = head;

free(temp);

return deleteEven(head->next);

}

else

{

head->next = deleteEven(head->next);

return head;

}

}

What is the purpose of heaving to set "head->next = deleteEven(head->next);" instead of just having "head = head->next;" in one of the lines above and just calling "deleteEven(head->next);"?

Answer 1

First of all there is a bug in the code, look at the code block of second if statement. After temp is assigned the address to which head is pointing, you have used free(temp). free( ) is used to free the memory location of the data so that it can be used by another program or function call. In some cases, the data is not immediately freed, and your program would work fine, since the address of next node can be found by using head->next. But in case the memory block is immediately used by another program or function call, then your program may crash because head is still pointing to this node which would now have some arbitrary data, and hence head->next would be some unknown memory location. Therefore, you should save head->next before using free.

Now let's analyse the correct code to answer your query.

Consider the following linked list (head = 100 is passed from main.)

Let's us look at the code with changes specified by you in block 3:

From head = 200, you directly passed head = 600 which means head = 400 would never be passed. Now at location 400, data can either be odd or even, this particular node would never be passed on , therefore breaking links. When you would backtrack, you would never be able to create links because they would be lost.

Introducing head = head->next is moving head one position and saving this new position in head. Now while calling deleteEven(head->next) you are again moving one position, so in all you moved two locations leaving one node unaddressed.

Therefore, the above code with mentioned corrections would work fine.

Hope it helps.