Segregate even and odd nodes in a Singly Linked List - singly-linked-list

Question

I am trying to modify the linked list such that all even numbers appear before all the odd numbers in the modified linked list. Also, keep the order of even and odd numbers same.
Below code works:
void segregateEvenOdd()
{
if(head==null) return;
Node temp=head,evenStart=null,evenEnd=null,oddStart=null,oddEnd=null;
while(temp!=null){
if(temp.data%2==0){
if(evenStart==null){
evenStart=temp;
//evenStart.next=null;
evenEnd=evenStart;
}
else{
evenEnd.next=temp;
evenEnd=evenEnd.next;
// evenEnd.next=null;
}
}
else{
if(oddStart==null){
oddStart=temp;
//oddStart.next=null;
oddEnd=oddStart;
}
else{
oddEnd.next=temp;
oddEnd=oddEnd.next;
//oddEnd.next=null;
}
}
temp=temp.next;
}
if(oddStart==null || evenStart==null) return;
evenEnd.next=oddStart;
oddEnd.next=null;
head=evenStart;
}
But when I uncomment the commented code, it doesn't work.
I am not able to understand this behavior.
Can someone explain?

Answer 1

In your code, evenStart, evenEnd, oddStart, oddEnd are just references which at starting are pointing to null but when it encounters first element(suppose Odd), then following code will run
if(oddStart==null){
oddStart=temp;
oddStart.next=null;
oddEnd=oddStart;
}
Here, in second line oddStart.next=null, it will not only change the next of oddStart but will also change the next of temp too. Due to which, if you uncomment everything your code will run the loop only once(because in first time temp next node will point to null).
Same case applies to all comments. You can print temp.data in loop to verify above fact.