#+ANKI_DECK: study_deck_02
* TODO 0091. Decode Ways :medium:
:PROPERTIES:
:NEETCODE: [[file:../../roadmap.org::*0091. Decode Ways][0091. Decode Ways]]
:END:

You have intercepted a secret message encoded as a string of numbers. The message is *decoded* via the following mapping:

~"1" -> 'A'

"2" -> 'B'

...

"25" -> 'Y'

"26" -> 'Z'~

However, while decoding the message, you realize that there are many different ways you can decode the message because some codes are contained in other codes (~"2"~ and ~"5"~ vs ~"25"~).

For example, ~"11106"~ can be decoded into:

 - ~"AAJF"~ with the grouping ~(1, 1, 10, 6)~

 - ~"KJF"~ with the grouping ~(11, 10, 6)~

 - The grouping ~(1, 11, 06)~ is invalid because ~"06"~ is not a valid code (only ~"6"~ is valid).

Note: there may be strings that are impossible to decode.

Given a string s containing only digits, return the *number of ways* to *decode* it. If the entire string cannot be decoded in any valid way, return ~0~.

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"). In this case, the string is not a valid encoding, so return 0.

*Constraints:*

 - ~1 <= s.length <= 100~

 - ~s~ contains only digits and may contain leading zero(s).

** TODO Approach
Write your approach here.

** TODO Python
#+begin_src python
class Solution:
    def numDecodings(self, s: str) -> int:
#+end_src

** TODO C++
#+begin_src cpp
class Solution {
public:
    int numDecodings(string s) {
        
    }
};
#+end_src