91. Decode Ways

public class Solution {
    public int numDecodings(String s) {
        if(s.length()==0)
            return 0;
        int[] dp=new int[s.length()+1];
        dp[0]=1;
        for(int i=0;i<s.length();i++)
        {
            if(s.charAt(i)>'0'&&s.charAt(i)<='9')
                dp[i+1]=dp[i];
            if(i>0)
            {
                int num=(s.charAt(i-1)-'0')*10+(s.charAt(i)-'0');
                if(9<num&&num<27)
                    dp[i+1]+=dp[i-1];
            }
        }
        return dp[s.length()];
    }
}