0092-reverse-linked-list-ii.py

class Solution:
    def reverseBetween(
        self, head: Optional[ListNode], left: int, right: int
    ) -> Optional[ListNode]:
        dummy = ListNode(0, head)

        # 1) reach node at position "left"
        leftPrev, cur = dummy, head
        for i in range(left - 1):
            leftPrev, cur = cur, cur.next

        # Now cur="left", leftPrev="node before left"
        # 2) reverse from left to right
        prev = None
        for i in range(right - left + 1):
            tmpNext = cur.next
            cur.next = prev
            prev, cur = cur, tmpNext

        # 3) Update pointers
        leftPrev.next.next = cur  # cur is node after "right"
        leftPrev.next = prev  # prev is "right"
        return dummy.next