Sword offer-Fifth chapter optimization of Time and space efficiency (total number of reversed pairs in an array)

Title: In an array if the preceding number of two digits is larger than the number that follows, it is called a pair of inverse pairs. Enter an array to find the total number of reverse pairs in the group.

Change Time by Space:
Idea: With an auxiliary array, copy the original array into the array. It then divides the array into sub-arrays, then counts the internal inverse of the subarray, and then counts the inverse pairs in the two connected sub-arrays, which uses the merge sort. The time complexity is O (NLOGN).

As shown in the following:

Sword offer-Fifth chapter optimization of Time and space efficiency (total number of reversed pairs in an array)