基于索引查找列表中大于条件的对数的最佳代码？

Question

Let's say A = [5,2,1,3], denote the number of pairs (i,j). 1<=i < j<=n such that A[i]>A[j]. Below is unoptimised code for same

def I(A):
    output = i = j = 0
    while i< len(A):
        j = i+1
        while j<len(A):
            if A[i]>A[j]:
                output +=1
            j+=1
        i+=1
    return output

Answer 1

Looks like your problem is the inversion count, very famous in competitive programming, the optimal solution to this is using mergesort, you can find plenty of implementations on the internet, I like the one in geeksforgeeks: https://www.geeksforgeeks.org/counting-inversions/

The basic idea behind it is using divide and conquer, the article above explains the problem in a very good way.