HackerRank - Insertion Sort Advanced Analysis

"How many inverted pairs" - that usually ends up with MergeSort solution (of course there are other solutions out there)

def mergeSort(arr):
if len(arr) == 1:
return 0, arr

mid = len(arr) // 2
cnt1, arr1 = mergeSort(arr[:mid])
cnt2, arr2 = mergeSort(arr[mid:])

#
ret = []
cnt = 0

i = 0
j = 0
inx = 0
while i < len(arr1) and j < len(arr2):
if arr1[i] <= arr2[j]:
ret.append(arr1[i])
i += 1
elif arr2[j] < arr1[i]:
ret.append(arr2[j])
cnt += len(arr1) - i        # arr2 -> arr1
j += 1
while i < len(arr1):
ret.append(arr1[i])
i += 1
while j < len(arr2):
ret.append(arr2[j])
j += 1

return cnt1 + cnt2 + cnt, ret

T = int(input())
for _ in range(0, T):
n = int(input())
arr = [int(i) for i in input().strip().split()]

cnt, _ = mergeSort(arr)
print (cnt)