Kth Smallest Element in a Sorted Matrix

The problem. Given an n × n matrix where every row and every column is sorted in ascending order, return the kth smallest value in the whole matrix — in sorted order, so duplicates count, not the kth distinct value.

Example

matrix = [[1, 5, 9], [10, 11, 13], [12, 13, 15]],  k = 8  ->  13
matrix = [[-5]],                                   k = 1  ->  -5
matrix = [[1, 2], [1, 3]],                         k = 2  ->  1     (the two 1s both count)