How to find the most common int in a 2d array of ints?

OK, so I'm just starting to think how to implement a new graphical plugin for Paint.NET and I will need to know how to find the most common integer in a 2d array of integers. Is there a built-in to C# way to do this? Or, does anyone have a slick way to do it?

The array will look something like this:

300 300 300 300 300 300 300
  0 150 300 300 300 300 300
  0   0 150 300 300 300 300
  0   0   0   0 300 300 300
  0   0   0   0 150 300 300
  0   0   0   0   0 150 300
  0   0   0   0   0   0 300

I would need to know that 300 is the most common number in the array. If there is no "most common" then just return the center number (the array dimintions will always be odd x odd) 0.

I'll be implementing this using a "brute force" algorithm unless you experts can come up with something faster.

Any help would be very much appreciated.

Thanks!

EDIT: More info...

The values will almost always be VERY diverse (more diverse than my example array). The values will be in the range of 0-360. The size of the array will be 5x5 to about 17x17 depending on speed of the algorithm. The result will be calculate once for each pixel in a large image... so faster is better. ;)

Solution

Take a look at the LocalHistogramEffect code in Paint.NET, notably LocalHistorgramEffect.RenderRect.

I walks the input image, maintaining a histogram of intensities for each source pixel withing 'r' pixels of the destination pixel. As the output pixels are traversed, it adds the leading edge to the histogram and subtracts the trailing edge. It handles all the edge cases well, and is quite fast. It's the basis for the Median, Unfocus, Outline, and Remove Noise effects.

Adapting this to support Hue instead of RGB intensity would be rather trivial.

The performance is quite good, and for your purposes it operates in O(r^2+wr+nw), where r is the radius, w is the width of the image, and n is the number of levels in the histogram.

-tjackson