On a N * N grid, we place some 1 * 1 * 1 cubes
that are axis-aligned with the x, y, and z axes.
Each value v = grid[i][j] represents a tower of v cubes
placed on top of grid cell (i, j).
Now we view the projection of these cubes onto the xy, yz, and zx planes.
A projection is like a shadow, that maps our 3 dimensional figure to a 2 dimensional plane.
Here, we are viewing the "shadow" when looking at the cubes from the top, the front, and the side.
Return the total area of all three projections.