794. Decode Ways

A message is encoded by mapping A -> 1, B -> 2, ..., Z -> 26. You need to count how many valid ways a numeric string can be decoded.

Given a string s containing only digits, return the number of possible decodings.

Examples

numDecodings('12');   // 2 -> 'AB', 'L'
numDecodings('226');  // 3 -> 'BBF', 'BZ', 'VF'
numDecodings('06');   // 0