Convert Binary Number in a Linked List to Integer LeetCode solution in Python

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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 getDecimalValue(self, head: ListNode) -> int:
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        res = 0
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        po = 0
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        stack = []
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        node = head
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        while node:
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            stack.append(node.val)
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            node = node.next
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        res = 0
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        for i in reversed(stack):
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            res += i * (2**po)
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            po += 1
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        return res