ass Solution {
    public int numDecodings(String s) {
        
        if(s == null || s.length() == 0)
            return 0;
        
        int [] dp = new int[s.length()+1];
        dp[0] = 1;
        dp[1] = s.charAt(0) != '0' ? 1:0;
        
        for(int i=2; i<=s.length(); i++)
        {
            int first = Integer.valueOf(s.substring(i-1, i));
            int second = Integer.valueOf(s.substring(i-2,i));
            
            if(first > 0 && first <= 9)
                dp[i] += dp[i-1];
            if(second > 9 && second <= 26)
                dp[i] += dp[i-2];
        }
        return dp[s.length()];
    }
}