Leetcode 2179. Count Good Triplets in an Array

Count the number of value triplets (x,y,z) in two permutations of 0..n−1 such that the indices of x,y,z are strictly increasing in both arrays (they form a common increasing subsequence of length three). With n up to 1e5, you need an efficient sub-quadratic counting approach rather than brute-force enumeration.