LeetCode 91 Decode Ways - Edge Cases

The tricky part: '$0$' cannot be decoded alone. There's no letter for $0$. If you're at a '$0$', you can't take one digit. You must have come from a valid two-digit combination ($10$ or $20$). "$06$" has $0$ ways (can't start with $0$). "$10$" has $1$ way (only "J"). "$27$" has $1$ way (only "BG", since $27$ > $26$).

These edge cases are where most people make mistakes. Take time to work through examples.