There are some methods that can make the solving easier and give you
a chance to test less cases. And there are some cases that and not
evident and need not only accurate coding but one bright idea to find
it. Before checking different cases you have to understand the
structure and logic of the program you are going to write (Not only
here - its common advice), after that the process of creating program
and finding bugs is not so complicated.
   Try again and long for success :)