"""
206. Reverse Linked List
Difficulty: Easy
https://leetcode.com/problems/reverse-linked-list/

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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?
"""


# Definition for singly-linked list.
# 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_ = curr.next
            curr.next = prev
            prev = curr
            curr = next_
        return prev