206.reverse-linked-list.py

#
# @lc app=leetcode id=206 lang=python3
#
# [206] Reverse Linked List
#
# https://leetcode.com/problems/reverse-linked-list/description/
#
# algorithms
# Easy (78.57%)
# Likes:    22526
# Dislikes: 505
# Total Accepted:    5M
# Total Submissions: 6.3M
# Testcase Example:  '[1,2,3,4,5]'
#
# Given the head of a singly linked list, reverse the list, and return the
# reversed list.
#
#
# Example 1:
#
#
# Input: head = [1,2,3,4,5]
# Output: [5,4,3,2,1]
#
#
# Example 2:
#
#
# Input: head = [1,2]
# Output: [2,1]
#
#
# Example 3:
#
#
# Input: head = []
# Output: []
#
#
#
# Constraints:
#
#
# The number of nodes in the list is the range [0, 5000].
# -5000 <= Node.val <= 5000
#
#
#
# Follow up: A linked list can be reversed either iteratively or recursively.
# Could you implement both?
#
#

# @lc code=start
# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
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]:
        prev = None
        curr = head
        while curr:
            next_node = curr.next
            curr.next = prev
            prev = curr
            curr = next_node
        return prev


# @lc code=end