Leetcode 1856. Maximum Subarray Min-Product

Find the maximum value of (minimum element of a contiguous subarray) × (sum of that subarray) over all non-empty subarrays and return it mod 1e9+7. Key insight: for each element treated as the subarray minimum, use a monotonic stack to find its maximal span and prefix sums to compute the sum efficiently (n up to 1e5).