Kth Smallest Element in a Sorted Matrix

DESCRIPTION (inspired by Leetcode.com)

You're given a square grid (n × n matrix) where each row is sorted in ascending order from left to right, and each column is also sorted in ascending order from top to bottom.

Given the matrix and an integer k, find the k-th smallest element among all elements in the matrix.

Note: k is 1-indexed, meaning k = 1 returns the smallest element.

Example 1:

Input:

matrix = [
    [ 1, 5, 9],
    [10,11,13],
    [12,13,15]]
k = 8

Output: 13

Explanation: The elements in sorted order are [1,5,9,10,11,12,13,13,15]. The 8th smallest element is 13.

Example 2:

Input:

matrix = [[-5]], k = 1

Output: -5

Explanation: The matrix has only one element, so it's the 1st smallest.