Here is a 3D representation of the 15 cases in the basic marching cube algorithm. Because it's easier to see what's happening in 3D more than in the 2D illustration of Wikipedia. See by yourself.
![See by yourself:](https://upload.wikimedia.org/wikipedia/commons/thumb/a/a7/MarchingCubes.svg/350px-MarchingCubes.svg.png)
I hope it will help you in your implementation of the marching cube :)