Algorithm for planarity test in graphs I am implementing a graph library and I want to include some basic graph algorithms in it. I have read about planar graphs and I decided to include in my library a function that checks if a graph is planar. I found on the web many efficient algorithms, but they all have the same drawback: they are very hard to implement. 
So here is my question: does there exist an algorithm for planarity checking which is easy to understand and to implement?

I think @K. Hu's suggestion of an algorithm based on Kuratowski's theorem must be the easiest to understand and implement.  Let's try to write it in pseudocode:
is_planar(G):
  If G is the empty graph, return TRUE.
  If G is isomorphic to K_(3,3) or K_5, return FALSE.
  For each vertex V of G:
    Let H be a copy of G with V and all adjacent edges removed.
    If not is_planar(H), return FALSE.
  For each edge E of G:
    Let H be a copy of G with E removed.
    If not is_planar(H), return FALSE.
  For each edge E of G:
    Let H be a copy of G with E contracted.
    If not is_planar(H), return FALSE.
  return TRUE.

This will typically only terminate in a reasonable amount of time for very small graphs.  However, there are optimizations such as memoization that can improve the running time somewhat without altering the overall structure of the program.  Possibly it could be extended to work for larger graphs of an unpathological nature, or at least those with certain nice properties.
Additionally, it has the benefit of generalizing easily to other topological surfaces as long as the forbidden minors are known.
