Runner technique to combine two equal Linked Lists

Question

So, I am facing a doubt here.

I was reading the book Cracking the coding Interview. The following text is written over there.

Suppose you had a linked list

Answer 1

Start p2 at second position.

a1(p1)-> a2 (p2) -> a3 -> a4 -> b1 -> b2 -> b3 -> b4  
a1-> a2 (p1) -> a3 -> a4 (p2)-> b1 -> b2 -> b3 -> b4  
a1-> a2  -> a3(p1) -> a4 -> b1 -> b2(p2) -> b3 -> b4  
a1-> a2  -> a3 -> a4(p1) -> b1 -> b2 -> b3 -> b4(p2)  

Answer 2

Let n = 2. We are starting with a list:

a1 -> a2 -> b1 -> b2

Let p1 be a "fast" pointer initially pointing to the successor of head.
Let p2 be a "slow" pointer initially pointing to the head.

      p1
a1 -> a2 -> b1 -> b2
p2

We move p1 by two and p2 by one until p1 reaches the end of the list (there is no next).

                  p1
a1 -> a2 -> b1 -> b2
      p2

Move p1 back to the head.

p1                  
a1 -> a2 -> b1 -> b2
      p2

Advance p2.

p1                  
a1 -> a2 -> b1 -> b2
            p2

"Weaving" starts.

Take element pointed by p2 and move it after p1. Advance p1 after inserted element.

            p1                  
a1 -> b1 -> a2 -> b2
                  p2

Take element pointed by p2 and move it after p1. Advance p1 after inserted element.

                       p1      
a1 -> b1 -> a2 -> b2  
                  p2

p1 is null, terminate.