Decode Ways

class Solution {
 private:
     vector<int> f;
 public:
     int getF(int index)
     {
         if (index < 0)
             return 1;
         else
             return f[index];
     }
     
     int numDecodings(string s) {
         // Start typing your C/C++ solution below
         // DO NOT write int main() function
         if (s.size() == 0)
             return 0;
             
         f.resize(s.size());
         
         for(int i = 0; i < s.size(); i++)
         {
             f[i] = 0;
             if (i >= 1)
             {
                 string a(s, i - 1, 2);
                 if ("10" <= a && a <= "26")
                     f[i] += getF(i-2);
                 if ('1' <= s[i] && s[i] <= '9')
                     f[i] += getF(i-1);
             }
             else
             {
                 if ('1' <= s[i] && s[i] <= '9')
                     f[i] = 1;
             }
         }
         
         return f[f.size() - 1];
     }
 };