Given a string containing only digits, return the number of ways to decode it where A=1, B=2, ..., Z=26.
Example 1:
Input: s = "12"
Output: 2
Explanation: "12" can be decoded as "AB" (1 2) or "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: "06" cannot be decoded — leading zeros are invalid.