Linked List

Pointer surgery. A dummy head removes edge cases around the real head.

Core syntax

• Dummy + tail — dummy = ListNode(); tail = dummy.
• Reverse — three pointers: prev, curr, nxt.

prev, curr = None, head
while curr:
    nxt = curr.next
    curr.next = prev
    prev = curr
    curr = nxt
return prev            # new head

Watch out

• Save curr.next before you overwrite it.
• Fast/slow pointers find the middle and detect cycles.