Reverse a Linked List

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Your question is Reverse a Linked List. Start with the requirements on the right.

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Problem

Given the head of a singly linked list, reverse the list and return the new head.

Implement a function that takes the head node of a linked list and reverses all next pointers so the list order is flipped. Do not create a new list of nodes.

Formal Specification

Input: head, the head node of a singly linked list, or None for an empty list.
Output: The head node of the reversed linked list.

Assume each node has two fields: val and next.

Constraints

The list may be empty.
0 <= number of nodes <= 10^5
-10^9 <= Node.val <= 10^9
Use O(1) extra space if possible.

Function Signature

def reverse_linked_list(head):

Your solutionPython 3

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