#linked_list

Question

Comments

here P!=NULL follows

because when q!=NULL , then p$\rightarrow$next!=NULL

So, means p!=NULL

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there are 2 node p points to 1st node and q to 2nd , we ignore the while(A) for some time , now if condition is false , now p points to 2nd node becoz of p=p-->next and then q-->next is null so it return q-->next to q means q=null if there is no cycle otherwise q pints to 1st node now come to condition while (A) its totally depend on q!=null

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Actually in linked list we cannot chk p!=NULL or q!=NULL

we have to chk p->next=NULL and q->next=NULL

So, if we chk q!=NULL that means nothing until we get p->next=NULL

That is why they also chk p value.

right? otherwise say where r u getting more doubt?

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@srestha

Actually in linked list we cannot chk p!=NULL or q!=NULL
we have to chk p->next=NULL and q->next=NULL

No, p!=NULL and q!=NULL are CORRECT since we initialize a node in any linked list with a Pointer and hence p and q are no doubt pointers and a pointer can be an address value or NULL

Note that p→next and q→next too are pointers so when p = p→next is executed then LSH(pointer) = RHS(pointer) and since you already know p→next is a pointer, therefore, p is also a pointer. Thus,

In linked list we can and we do check p!=NULL and/or q!=NULL

Answer 1

You are correct.. for `p` to ever be NULL, there needs to be an empty list and q!=NULL already checks for that

BUT

When the given list is empty (head = NULL), NULL->next has UNDEFINED behaviour, this means that when the 2nd line of code

Q = head→next               //becomes

Q = NULL→next .

Then this code will either throw error (Compiler) or Segmentation fault or (if it is/isn't run in "X" language) it can (it shouldn't) still assign a value to Q .

 If it does assign a value then the while code will execute and if the code is 

while ( q!=NULL )

 This can lead the loop to execute forever just because a "series" of UNDEFINED behaviour "can" occour. I am strongly  emphasising on the words "series" and "can".

Hence the accurate code is infact

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

i.e. option C

Comments

but our goal is to find cycle so atleast 2 node will be there

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Our goal is to find cycles in ANY type of Linked Lists... which can be {acyclic , cyclic , empty}

Always remember that algorithms always check for set of all possible inputs which include the empty inputs also so that the algorithm is error free...

Hope this helps....

 

BTW you can query any difficulties you have in this or any program (programming only)and can go in complete details(full nerd) with me ... I too am fan of people's logic