Given an integer array, count the number of inversions.

Inversion count is the distance of order of elements in an array from the the sorted order i.e. it indicates how far (or close) the array is from being sorted. If array is already sorted then inversion count is 0. If array is sorted in reverse order that inversion count is the maximum.
In other words, two elements a[i] and a[j] form an inversion if a[i] > a[j] and i < j