Problem

Reverse a singly linked list.
Input: 1->2->3->4->5->NULL
Output: 5->4->3->2->1->NULL

Solution

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    def reverseList(self, head: ListNode) -> ListNode:
        prev = None
        curr = head
        while curr:
            next = curr.next
            curr.next = prev
            prev = curr
            curr = next
        return prev