Leetcode 2916. Subarrays Distinct Element Sum of Squares II

Compute the sum over all nonempty subarrays of (number of distinct elements)^2 in an integer array, returning the result modulo 1e9+7. With n up to 1e5, the core challenge is to avoid O(n^2) enumeration by algebraically expanding the square into per-element and pairwise contributions and counting those efficiently using occurrence tracking.