First Missing Positive LeetCode solution in Python

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# Runtime: 295 ms (Top 90.42%) | Memory: 28 MB (Top 72.11%)
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class Solution:
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    def firstMissingPositive(self, nums: List[int]) -> int:
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        mn = float("inf")
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        mx = 0
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        numsSet = set()
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        for i in range(len(nums) - 1, -1, -1):
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            if nums[i] > 0:
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                if nums[i] < mn:
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                    mn = nums[i]
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                if nums[i] > mx:
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                    mx = nums[i]
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                numsSet.add(nums[i])
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            del nums[i]
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        if mn >= 2:
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            return 1
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        if len(numsSet) == mx:
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            return mx + 1
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        for i in range(2, len(numsSet) + 1):
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            if i not in numsSet:
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                return i