Leetcode 363. Max Sum of Rectangle No Larger Than K

Find the maximum-sum subrectangle in an m×n matrix whose sum is no larger than k. The core challenge is reducing the 2D problem to many 1D "max subarray ≤ k" problems by fixing two boundaries and using prefix sums with an ordered set to quickly find the best subarray; iterate over the smaller dimension if rows ≫ columns.