Kth Smallest Number in Sorted Matrix

Problem

Find the kth smallest number in at row and column sorted matrix.

Example

Given k = 4 and a matrix:

[
  [1 ,5 ,7],
  [3 ,7 ,8],
  [4 ,8 ,9],
]

return 5

Challenge

O(k log n), n is the maximal number in width and height.