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How can the runner technique be used to arrange this linked list?

  1. Dec 19, 2013 #1
    1. The problem statement, all variables and given/known data

    POpQu.png

    2. Relevant equations



    3. The attempt at a solution

    I don't understand how when p1 hits the end of the linked list, p2 will be at the midpoint.

    This is how I am imagining it if n=3. Each step below represents an iteration.

    1. a1 (p1, p2)->a2->a3->b1->b2->b3

    2. a1->a2 (p2)->a3 (p1)->b1->b2->b3

    3. a1->a2->a3 (p2)->b1->b2 (p1)->b3

    4. Index out of bounds error because p2 now points to a node after b3.
     
  2. jcsd
  3. Dec 19, 2013 #2

    jedishrfu

    Staff: Mentor

    It says you don't know the list length but you do know that its length is even.

    Hence you can't have n=3 in your thought experiment.

    So if n=10 then:

    i=0 p1=0 p2=2
    i=1 p1=1 p2=4
    i=2 p1=2 p2=6
    i=3 p1=3 p2=8
    i=4 p1=4 p2=0 // p2 is reset to the beginning of the list and p1 is at the midpoint
     
  4. Dec 19, 2013 #3
    Oh, so I guess they wanted n to be even. Even if n is odd, the linked list length is even since the length of the linkedlist is 2n.
     
  5. Dec 19, 2013 #4

    jedishrfu

    Staff: Mentor

    Only the linked list length is even, if n is half the list length then n can be even or odd.
     
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