Decode Ways

A message made of the capital letters A to Z was turned into digits by writing each letter as its position in the alphabet: A becomes 1, B becomes 2, and so on up to Z, which becomes 26.

To decode the digits, split them into groups, each group a number from 1 to 26, and turn each group back into its letter. A group can't start with 0: "06" is not the same as "6", and no letter is 0. For example, "11106" can be decoded in two ways:

• "AAJF", from the groups 1, 1, 10, 6;
• "KJF", from the groups 11, 10, 6.

The groups 1, 11, 06 don't work, because 06 starts with 0.

Given a string s of digits, return the number of ways to decode it. If it can't be decoded at all, return 0.

Example 1:

Input: s = "12"
Output: 2
Explanation: "12" can be decoded as "AB" (1, 2) or as "L" (12).

Example 2:

Input: s = "226"
Output: 3
Explanation: "226" can be decoded as "BZ" (2, 26), "VF" (22, 6) or "BBF" (2, 2, 6).

Example 3:

Input: s = "06"
Output: 0
Explanation: The first group would have to start with 0: "0" is no letter, and "06" can't be read as 6. So "06" can't be decoded.

Constraints:

• 1 <= s.length <= 100
• s is made of digits only, 0 to 9, and may start with 0.
• The answer fits in a 32-bit integer.