Question

I am looking for a method that reverses the same instance of a given list, with O(1) additional space and O(n) time.

this is not HW nor I am looking for some library method to do the job for me, as this is only an exercise for myself, and out of pure curiousity.

any ideas how to do it with O(1) additional space and O(n) time? (and if possible without reflection as well)?

signature is public <T> void reverse(List<T> list).

(*)assume get() to the head and tail of the list is O(1), but to the middle of it is O(n).

I came up with a recursive solution, but it is O(n) space, O(n) time

public <T> void reverseAux(List<T> list,int size) {
    if (size == 0) return;
    T elem = list.remove(size-1);
    reverseAux(list,size-1);
    list.add(0,elem);
}
public <T> void reverse(List<T> list) {
    reverseAux(list, list.size());
}

EDIT: I am looking for a java solution, for List<T>, only assumption on implementation is access time O(1) for head and tail, and using List<T> interface.

Answer 1

Just read one of the following. It is the thing you’re talking about.

Please note that we’re talking about singly ‘linked’ lists.

http://www.teamten.com/lawrence/writings/reverse_a_linked_list.html

http://www.mytechinterviews.com/reverse-a-linked-list

http://www.geekpedia.com/code48_Reverse-a-linked-list.html

http://www.codeproject.com/KB/recipes/ReverseLinkedList.aspx

Plus an extra question for you:

How would you find Nth element from the tail of a linked list assuming it is singly linked and you have only head pointer with O(1) space and O(N) time?