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.

The test cases are generated so that the answer fits in a 32-bit integer.

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 ("6" is different from "06").

Constraints:

- `1 <= s.length <= 100`
- `s` contains only digits and may contain leading zero(s).