Count inversions
Given an integer array, count the number of inversions: pairs of indices (i, j) with i < j and arr[i] > arr[j]. Values may be large and negative, so coordinate-compress them and use a Fenwick (binary indexed) tree to count, for each element processed left to right, how many already-seen elements are strictly greater. Return the total inversion count. The array length is up to 1e5, so an O(n log n) solution is expected (O(n^2) will time out).
Implement
count_inversions(arr: list[int]) → intExamples
in
[[2,3,8,6,1]]out5What a strong answer looks like
State your approach and its time/space complexity out loud before you optimize. Handle the edge cases (empty input, duplicates, overflow), and say why you chose this over the brute force. Green tests are the floor, not the grade.
0:00 of about 30 min
solution.py
InputExpectedGot
[[2,3,8,6,1]]5not run yetsample