title: Kth smallest element in sorted matrix
body: <p>This is an interview question.   </p>
  
  <p>Find the K<sup>th</sup> smallest element in a matrix with sorted rows and columns.<br>
  Is it correct that the K<sup>th</sup>  smallest element is one of <code>a[i, j]</code> such as <code>i + j = K</code> ? </p>