Note that it is the kth smallest element in the sorted order, not the kth distinct element.
 Example:
 Input:
 matrix = [
 [ 1, 5, 9],
 [10, 11, 13],
 [12, 13, 15]
 ],
 k = 8,
 Output: 13
 Note:
 You may assume k is always valid, 1 ≤ k ≤ n^2. You may also assume that 1 <= n <= 1000.