Leetcode 3070. Count Submatrices with Top-Left Element and Sum Less Than k

Count how many submatrices that include the top-left cell (0,0) have sum ≤ k in an m×n matrix of nonnegative integers. Use the non-negativity and prefix sums (or two-pointer/monotonic expansion over bottom-right corners) to efficiently enumerate valid rectangles for m,n up to 1000.