Encrypt and Decrypt Strings

You're building a tiny cipher where each letter becomes a two-letter code, then answering the reverse question: how many approved words could have produced this coded string? Interviewers love it because the two directions look symmetric until you try to make the second one fast. The classic trap is assuming every code points back to exactly one letter. Don't let the Hard label intimidate you; the core idea is simpler than it looks.

Meet Coco

Coco is an AI study partner, not a person. She listens while you explain your code, asks what a line does and never hands you the fix.