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). For example, `"11106"` can be mapped into: - `"AAJF"` with the grouping `(1 1 10 6)` - `"KJF"` with the grouping `(11 10 6)` Note that the grouping `(1 11 06)` is invalid because `"06"` cannot be mapped into `'F'` since `"6"` is different from `"06"`. Given a string `s` containing only digits, return the number of ways to decode it. If there are no valid decodings, return `0`. **Example 1:** ``` Input: s = "12" Output: 2 Explanation: "12" could be decoded as "AB" (1 2) or "L" (12). ``` **Example 2:** ``` Input: s = "226" Output: 3 Explanation: "226" could be decoded as "BZ" (2 26), "VF" (22 6), or "BBF" (2 2 6). ``` **Example 3:** ``` Input: s = "06" Output: 0 Explanation: "06" cannot be mapped to "F" because of the leading zero. ```