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This article mainly introduces LeetCode how to find the reverse pairs in the array, the article is very detailed, has a certain reference value, interested friends must read it!

Topic description

Two numbers in an array that form a reverse pair if the first number is greater than the next. Enter an array to find the total number of reverse pairs in this array.

0 = e:

Return

M = (s + e) > > 1

# sort and count the left and right parts first

Mergesort (s, m)

Mergesort (m + 1, e)

# use temporary arrays to store sorted left and right parts

# Note that reverse storage is selected here because the smallest element can be obtained by using the pop operation of O (1).

# of course, double-ended queues can also be used here, so that there is no need to reverse order

Left = nums [svm + 1] [::-1]

Right = nums [m + 1 e + 1] [::-1]

For i in range (s, e + 1):

If not right or left and left [- 1]

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