Solution.java

public class Solution {
    public int numDecodings(String s) {
        if (s.length()==0) {
            return 0;
        }
        Map<String,Integer> map=new HashMap<String, Integer>();
        return f(s,map);
    }
    public int f(String s,Map<String,Integer> map){
        if (s.length()==0) {
            return 1;
        }
        if (map.get(s)==null) {
            int length=s.length();
            int one=-1;
            if (length>=1) {
                one=Integer.parseInt(s.substring(0,1));
            }
            int two=-1;
            if (length>=2) {
                two=Integer.parseInt(s.substring(0,2));
            }
            int value=(one>0?f(s.substring(1,s.length()),map):0)+((one>0&&two>0&&two<27)?f(s.substring(2,s.length()),map):0);
            map.put(s, value);
            return value;
        }
        return map.get(s);
    }
}