Watering the farm
There is a farmer, he wants to water his farm by arrange his trees.
His farm has an N by M square. Each tree of his farm is 1 by 1 square. Each of it has its own height . His job is to separate the NxM farm square into some 2x2 tree square mark it from Top to Bottom, Left to Right. Arrage the height of the NxM farm's trees greater than. In each 2x2 square put the higest tree from the Top to the Bottom, the Left to the Right. Put it back the NxM square farm.