Add Two Numbers II LeetCode solution in Python

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class Solution:
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    def addTwoNumbers(
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        self, l1: Optional[ListNode], l2: Optional[ListNode]
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    ) -> Optional[ListNode]:
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        def number(head):
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            ans = ""
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            while head:
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                ans += str(head.val)
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                head = head.next
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            return int(ans)
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        temp = dummy = ListNode(0)
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        for i in str(number(l1) + number(l2)):
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            temp.next = ListNode(i)
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            temp = temp.next
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        return dummy.next