Encode or Decode Message

Problem Overview

• Encode or decode a message using a shared numeric key; apply key digits cyclically.
• Inputs/outputs: operation (1 encode, 2 decode), message string, positive integer key; return string or -1 for invalid input or mismatched patterns.
• Constraints: if key ends first, remaining characters stay unchanged when encoding; decoding must validate repeats; nonconformance returns -1.
• Context: String transformation for a coding interview; a classic interview question problem.