Decode Ways

A string consisting of uppercase english characters can be encoded to a number using the following mapping:

'A' -> "1"
'B' -> "2"
...
'Z' -> "26"

To decode a message, digits must be grouped and then mapped back into letters using the reverse of the mapping above. There may be multiple ways to decode a message. For example, "1012" can be mapped into:

• "JAB" with the grouping (10 1 2)
• "JL" with the grouping (10 12)

The grouping (1 01 2) is invalid because 01 cannot be mapped into a letter since it contains a leading zero.

Given a string s containing only digits, return the number of ways to decode it. You can assume that the answer fits in a 32-bit integer.

"12" could be decoded as "AB" (1 2) or "L" (12).

"01" cannot be decoded because "01" cannot be mapped into a letter.