Question: Given an arbitrary two dimensional matrix of integers where the elements are sorted in increasing order along rows and columns, find a number in the matrix closest to and less than or equal to a given number.
Answer:
 uint GetElement(int [,] matrix, uint startrow, uint startcol, uint endrow, uint endcol, uint number)
{
while(startrow < endrow && startcol < endCol)
{
uint midrow = (startrow + endrow) / 2 ;
uint midcol = (startcol + endcol) / 2;

if (matrix[midrow, midcol] < number))
{
startrow = midrow;
startcol = midcol;
}
else
{
endrow = midrow;
endcol = midcol;
}
}
if (startrow == endrow && startcol == endcol)
{
  return matrix[startrow, startcol] < number ? matrix[startrow, startcol] : 0;
}
if ((startcol == endcol && startrow == endrow - 1) || (startrow == endrow && startcol == endcol - 1) )
{
  if (matrix[endrow, endcol] < number) return matrix[endrow, endcol];
  if (matrix[startrow, startcol] < number) return matrix [ startrow, startcol];
  return 0;
}
if (matrix[startrow, startcol] < number)
{
startrow = endrow;
startcol = endcol;
}
uint topright =  startcol - 1 > 0 && startrow - 1 > 0  ? GetElement(matrix, 0, startcol, startrow - 1, endcol, number) : 0;
uint bottomleft = startrow + 1 <= endrow && startcol - 1 > 0 ? GetElement(matrix, startrow + 1, 0, endrow, startcol - 1,
number) : 0;
if (topright < bottomleft)
  return bottomleft;
else
  return topright;
}