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.

class Solution {
public:
    int numDecodings(string s) {
        if(s.empty() || s[0] == '0') return 0;
        int len = s.length();
        if(len == 1) return check(s[0]);
        int fn = 0;
        int fn1 = 1;
        int fn2 = check(s[0],s[1]) + check(s[1]);
        for(int i=2; i<len; i++)
        {
            if(check(s[i])) fn += fn2;
            if(check(s[i-1],s[i])) fn += fn1;
            fn1 = fn2;
            fn2 = fn;
            fn = 0;
        }
        return fn2;
    }
    int check(char x)
    {
        return (x != '0') ? 1:0;
    }
    int check(char x, char y)
    {
        return (x == '1' || x == '2' && y <= '6') ? 1:0;
    }
};