Decode Ways

A message containing letters from `A-Z` can be encoded into numbers using the following 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 above (there may be multiple ways). Given a string `s` containing only digits, return the number of ways to decode it. The test cases are generated so that the answer fits in a 32-bit integer.