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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 deleteDuplicates(self, head: Optional[ListNode]) -> Optional[ListNode]:
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        if not head:
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            return None
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        result = tail = ListNode(-1)
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        while head:
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            curr = head
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            head = head.next
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            hasDup = False
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            while (head) and (curr.val == head.val):
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                hasDup = True
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                headNext = head.next
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                head = None
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                head = headNext
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            if hasDup == False:
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                tail.next = curr
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                tail = tail.next
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                tail.next = None
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        return result.next