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# Runtime: 71 ms (Top 18.71%) | Memory: 15.5 MB (Top 28.33%)
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# Definition for singly-linked list.
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# class ListNode:
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# def __init__(self, val=0, next=None):
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# self.val = val
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# self.next = next
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class Solution:
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    def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]:
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        if head == None or head.next == None:
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            return head
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        p = None
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        while head != None:
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            temp = head.next
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            head.next = p
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            p = head
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            head = temp
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        return p