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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]))
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