Why does my Python linked-list reversal loop fail to terminate?

With a list such as 1 -> 2 -> 3, setting p2.next = p1 changes the link to 2 -> 1, but node 1 still points to node 2. That creates a cycle between the first two nodes. You also set p2 to temp.next rather than simply advancing it to the saved next node, which skips nodes and can eventually dereference None.

If you want to keep the two-pointer style, detach the old head first and advance through the saved node:

if head is None:
return None

p1 = head
p2 = head.next
p1.next = None

while p2 is not None:
temp = p2.next
p2.next = p1
p1 = p2
p2 = temp

return p1

There is also a small safety issue before the loop: p2 = head.next is executed before checking whether head is None. An empty list would therefore raise an AttributeError. Check head first, or use the standard prev/current approach, which naturally handles both an empty list and a one-node list. For comparisons with None, prefer `is None` and `is not None` rather than `== None` and `!= None`.

The usual iterative solution keeps three references: the previous node, the current node, and the next node. Save current.next before changing it, point current backward, then advance both pointers. The new head is prev, not the original head:

prev = None
curr = head

while curr is not None:
nxt = curr.next
curr.next = prev
prev = curr
curr = nxt

return prev

Your code changes p2.next before safely preserving the rest of the list, and it also returns the old head instead of the new one.