[51Qs] Reverse Linked List

	# Leetcode #206
	# Definition for singly-linked list.
	# class ListNode:
	# def __init__(self, val=0, next=None):
	# self.val = val
	# self.next = next
	"""
	head
	\|
	IN 1->2->3->4->Null

	OUT
	Null<-1<-2<-3<-4

	Intermediate State1
	** *
	1<-2 3->4->Null
	\| \| \
	prev curr nextTemp

	Intermediate State2
	1<-2<-3 4->NULL
	\| \|
	prev curr
	* **
	where *: nextTemp=curr.next(old) # from [3->4] to [3 4]
	where **: curr.next(new)=prev(old) # from [2 3] to [2<-3], since prev=2, curr=3
	where ***: prev(new)=curr(old) # from [prev=2] to [prev=3]
	where ****: curr(new)=nextTemp # from [curr=3] to [curr=4]
	"""
	class Solution:
	def reverseList(self, head: ListNode) -> ListNode:
	prev = None
	curr = head
	while curr:
	nextTemp = curr.next
	curr.next = prev
	prev=curr
	curr = nextTemp
	return prev

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