Missing Number LeetCode solution in Python

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class Solution:
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    def missingNumber(self, nums: List[int]) -> int:
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        # T.C = O(n) S.C = O(1)
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        actualsum = 0
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        currentsum = 0
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        i = 1
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        for num in nums:
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            currentsum += num
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            actualsum += i
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            i += 1
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        return actualsum - currentsum