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Understand the problem
Restate the problem precisely. Interviewers reward candidates who pause, define the contract, and name the guarantees before coding.
A message containing letters from A-Z can be encoded into numbers using the mapping 'A'→"1", 'B'→"2", …, 'Z'→"26". To decode an encoded message, all the digits must be grouped, then mapped back into letters using the reverse of the mapping. Note that '06' is **not** a valid encoding of 'F'. Given a string `s` containing only digits, return the number of ways to decode it. The answer is guaranteed to fit in a 32-bit integer.
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