206_reverse_linked_list.py

#
# @lc app=leetcode id=206 lang=python3
#
# [206] Reverse Linked List
#

# @lc code=start
# Definition for singly-linked list.
from typing import Optional


class ListNode:
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next


class Solution:
    def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]:
        if not head or not head.next:
            return head
        left, right = head, head.next
        left.next = None

        while right:
            next = right.next
            right.next = left
            left = right
            right = next

        return left


# @lc code=end