A subsequence is created by deleting zero or more elements from a list while maintaining the order. For example, the subsequences of `[1,2,3]`

are `[1]`

, `[2]`

, `[3]`

, `[1,2]`

, `[1,3]`

, `[2,3]`

, `[1,2,3]`

. An inversion is a strictly decreasing subsequence of length 3. More formally, given an array `p=p[n]`

an inversion in the array is any time some `p[i]>p[j]>p[k]`

and `i<j<k`

.

This is a companion discussion topic for the original entry at https://algo.monster/problems/citadel-oa-inversions/