The tricky part: '' cannot be decoded alone. There's no letter for . If you're at a '', you can't take one digit. You must have come from a valid two-digit combination ( or ). "" has ways (can't start with ). "" has way (only "J"). "" has way (only "BG", since > ).
These edge cases are where most people make mistakes. Take time to work through examples. The pattern becomes clearer with practice.