Leetcode 1420. Build Array Where You Can Find The Maximum Exactly K Comparisons

Count the number of length-n arrays with values in [1, m] such that, when scanning left-to-right, a new maximum is found exactly k times (i.e., there are k record-breaking elements), returning the result modulo 10^9+7. This is a combinatorics / dynamic-programming counting problem over record highs.