Number of ways decoding a message (Decode Ways)

A message containing letters from A-Z is being encoded to numbers using the following mapping:

‘A’ -> 1
‘B’ -> 2
…
‘Z’ -> 26

Given an encoded message containing digits, determine the total number of ways to decode it.

For example, given encoded message “12”, it could be decoded as “AB” (1 2) or “L” (12). The number of ways decoding “12” is 2.

Thoughts:
Again, we can use recursion with backtracking. Each time we try to encode the first digit in the message to a letter, or we can encode the first two digits into a letter if possible, we then recursively do the samething for the sub-string. When there is no way to encode (e.g., encountering a single ‘0’, or encountering ’32’ and tyring to encode the two-digits to a letter), we simply return. If there is no sub-string to go on, we know we have found a valid solution, so we increase the count.

Code (C++):

class Solution {
public:
    int numDecodings(string s) {
        int solutions = 0;
        if(s.size() != 0)
            recursion(s, solutions);
        return solutions;
    }
    
    void recursion(string s,int &solutions) {
        if(s.size() == 0){
            solutions++;
        } else {
            int value = (int)s.at(0)- '0';
            if(value > 0 && value <= 9) {
                recursion(s.substr(1), solutions);
            }
            
            if(s.size() >= 2) {
                int value1 = (int)s.at(0) - '0';
                int value2 = (int)s.at(1) - '0';
                if(value1 == 1 && value2 >= 0 && value2 <= 9) {
                    recursion(s.substr(2), solutions);
                } else if(value1 == 2 && value2 >= 0 && value2 <= 6) {
                    recursion(s.substr(2), solutions); 
                }
            }
        }
    }
};

Code (Java):

This is a very werid way to do it, I know. The ArrayList solutions are pretty much useless there other than counting the number of valid solutions. It’s just a way to bypass the fact that Java cannot pass argument by reference.
import java.util.ArrayList;
public class Solution {
    public int numDecodings(String s) {
        ArrayList<Integer> solutions = new ArrayList<Integer>();
        if(s.length() != 0)
            recursion(s, solutions);
        return solutions.size();
    }
    
    private void recursion(String s, ArrayList<Integer> solutions) {
        if(s.length() == 0){
            solutions.add(1);
        } else {
            int value = Character.getNumericValue(s.charAt(0));
            if(value > 0 && value <= 9) {
                recursion(s.substring(1), solutions);
            }
            
            if(s.length() >= 2) {
                int value1 = Character.getNumericValue(s.charAt(0));
                int value2 = Character.getNumericValue(s.charAt(1));
                if(value1 == 1 && value2 >= 0 && value2 <= 9) {
                    recursion(s.substring(2), solutions);
                } else if(value1 == 2 && value2 >= 0 && value2 <= 6) {
                    recursion(s.substring(2), solutions); 
                }
            }
        }
    }
}