Count Pairs With XOR in a Range LeetCode solution in Python

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class Solution:
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    def countPairs(self, nums, low, high):
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        return self._countPairs(nums, high + 1) - self._countPairs(nums, low)
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    def _countPairs(self, nums, high):
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        res = 0
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        for k in range(31, -1, -1):
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            target = high >> k
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            if target & 1 == 0:
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                continue
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            target -= 1
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            counter = Counter(num >> k for num in nums)
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            for mask in counter:
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                res += counter[mask] * counter[target ^ mask]
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                if mask == target ^ mask:
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                    res -= counter[mask]
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        return res // 2