Data Structure

Question

p=head;
q=head-> next;
while(A)
{
     .......................................
}

A is the condition to see list is empty or not, which one is valid?

a) p!=NULL;

b) q!=NULL;

c)(p!=NULL)&&(q!=NULL)

d)(p!=NULL)||(q!=NULL)

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a) p!=NULL;

b) q!=NULL;

c)(p!=NULL)&&(q!=NULL)

d)(p!=NULL)||(q!=NULL)

Comments

Wha is the code within while() loop?

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Whatever the code is.

I just want to get a condition checking for A, to see list is empty or not

For that which option is suitable?

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We have options in terms of q as well, so we can't say it independently of the code. 

head can be implemented in two ways:

1) which contains the data as well

2) which just acts as a pointer to first data node.

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I want to see if list is empty or not

Here q is next node of head i.e. p.

Then what will be the code?

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while(p!=null){

...

}

Here while loop won't execute if list is empty as head i.e p will be null. 

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if list is nul  then 

• head = null
• head -> next = null // depends on how head -> next is initialised .

if list contains one node 

• head = node
• head -> next = null 

in the 2nd case option B , C ,D could be true . But list is actually Not null.  

hence option A is the answer

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Added the last part. Say what will be answer now and why?

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Should be option B

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how, explain in detail

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Option B is correct.

If q!=Null then p is definitely not equal to Null.

The code must not enter the body of while loop if q=Null, this is because,

Observe this statement,

p = (p->next)? (p->next->next) : (p->next);

if at all p=Null then the condition (p->next) gives segmentation fault as it tries to access the next of Null.

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@shreya here p moves one by one, while q moves two at a time whenever q is not pointing to last node... so if list will end q can detect it easily [q!=NULL] but the above code will work only if list has atleast one element..consider empty list... then p-> head [which is null for empty list] and q = head->next ..so now while loop condition should give runtime error as q is saying to point next of NULL.

Answer 1

List is empty means there would not be any node in the list. Means the pointer "head" should point to Null.

'q' will be considered if there exist atleast a single node. So all other three option will be eliminated.(As question is talking about empty list.)

Option A is correct.