how to reverse a list with O(1) space and O(n) time?

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 me

Answer 1

The best performance you can get from comparison sorts like merge sort or quick sort is O(nlogn). You can get O(n) performance from non-comparison sorts like radix sort.

If you are reversing a linked-list, then you can reverse the list in O(n) time with using just 3 extra items. You need 3 pointers to keep track of what you're currently pointing to, what is before your current item and what is after your current item. The code is:

Node current = head;
Node next = null;
Node prev = null;
while (current != null) {
    next = current.next;
    current.next = prev;
    prev = current;
    current = next;
}
return prev;