Ways to Split Array Into Three Subarrays

Count the number of ways to choose two split indices to partition the array into three non-empty contiguous subarrays (left, mid, right) such that sum(left) ≤ sum(mid) ≤ sum(right). Because nums are non-negative, prefix sums are non-decreasing, so you can use two pointers or binary searches on prefix sums to count valid mid-split ranges for each left split (answer mod 1e9+7).