LeetCode 907 Sum of Subarray Minimums - Why Naive Fails

Generate all $O(n^2)$ subarrays. For each, find the minimum in $O(n)$ time.

Total: $O(n^3)$. Even optimizing to $O(n^2)$ (tracking min while extending) is too slow for $n = 30000$.

Can you count each element's contribution in $O(n)$?