Reversed Linked List: Why does the position of head = head.next matter?

Question

Can anyone explain why the position of this line head=head.next must be right after curr=head and not any further down? Initially, I was putting head=head.next in the last line of the while loop but was getting only the last node. Eg. If the input was [1,2,3] , I would get [1] using the second block of code and [3,2,1] with the first.

Correct:

def reverseListIterative(head: ListNode):
    prev = None
    while head:
        curr = head
        head = head.next
        curr.next = prev
        prev = curr
    return prev

Wrong:

def reverseListIterative(head: ListNode):
    prev = None
    while head:
        curr = head
        curr.next = prev
        prev = curr
        head = head.next
    return prev

Answer 1

You could figure this out by tracking the content of each variable through the code flow (you don't even need to run it):

Let's say we start with the list 1 -> 2 -> 3

                          prev  curr head  head.next  curr.next
                          ----  ---- ----  ---------  --------- 
prev = None               None  -    1     2          -
while head:
    curr = head           None  1    1     2          2  
    curr.next = prev      None  1    1     None       None      *** 
    prev = curr           1     1    1     None       None 
    head = head.next      1     1    None  -          None    
return prev

When you assign prev (None) to curr.next, you lose the connection from head.next because head is also pointing to node #1.