206. Reverse Linked List¶

#
# @lc app=leetcode id=206 lang=python3
#
# [206] Reverse Linked List
#
# https://leetcode.com/problems/reverse-linked-list/description/
#
# algorithms
# Easy (73.66%)
# Likes:    19643
# Dislikes: 357
# Total Accepted:    3.4M
# Total Submissions: 4.6M
# Testcase Example:  '[1,2,3,4,5]'
#
# Given the head of a singly linked list, reverse the list, and return the
# reversed list.
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#
# Example 1:
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# Input: head = [1,2,3,4,5]
# Output: [5,4,3,2,1]
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# Example 2:
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# Input: head = [1,2]
# Output: [2,1]
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#
# Example 3:
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# Input: head = []
# Output: []
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#
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# Constraints:
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# The number of nodes in the list is the range [0, 5000].
# -5000 <= Node.val <= 5000
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#
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# Follow up: A linked list can be reversed either iteratively or recursively.
# Could you implement both?
#
#

# @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]:

        pre = None
        cur = head

        while cur:
            tmp = cur
            cur = cur.next

            tmp.next = pre
            pre = tmp

        return pre

# @lc code=end