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+# 493. Reverse Pairs
+
+"""
+given an integer array 'nums', return the number of reverse pairs in the
+array. a reverse pair is a pair '(i, j)' where '0 <= i < j < nums.len' and
+'nums[i] > 2 nums[j]'
+"""
+
+
+class Bits:
+ def __init__(self, n):
+ self.n = n + 1
+ self.sums = [0] * self.n
+
+ def update(self, i, delta):
+ while i < self.n:
+ self.sums[i] += delta
+ i += i & (-i)
+
+ def query(self, i):
+ res = 0
+ while i > 0:
+ res += self.sums[i]
+ i -= i & (-i)
+ return res
+
+
+class Solution(object):
+ def reversePairs(self, nums):
+ """
+ :type nums: List[int]
+ :rtype: int
+ """
+ new_nums = nums + [x * 2 for x in nums]
+ sorted_set = sorted(list(set(new_nums)))
+ tree = Bits(len(sorted_set))
+ ans, ranks = 0, {}
+ for i, n in enumerate(sorted_set):
+ ranks[n] = i + 1
+ for n in nums[::-1]:
+ ans += tree.query(ranks[n] - 1)
+ tree.update(ranks[n * 2], 1)
+ return ans
+
+
+if __name__ == "__main__":
+ obj = Solution()
+ print(obj.reversePairs(nums=[1, 3, 2, 3, 1]))
+ print(obj.reversePairs(nums=[2, 4, 3, 5, 1]))