decode.java

import java.util.*;

public class decodingcountDP {

    static int calls = 0;    
    static Map<Integer, String> codes  = new HashMap<Integer, String>();

    private static void construct(){
    codes.put(1, "A");
    codes.put(2, "B");
    codes.put(3, "C");
    codes.put(4, "D");
    codes.put(5, "E");
    codes.put(6, "F");
    codes.put(7, "G");
    codes.put(8, "H");
    codes.put(9, "I");
    codes.put(10, "J");
    codes.put(11, "K");
    codes.put(12, "L");
    codes.put(13, "M");
    codes.put(14, "N");
    codes.put(15, "O");
    codes.put(16, "P");
    codes.put(17, "Q");
    codes.put(18, "R");
    codes.put(19, "S");
    codes.put(20, "T");
    codes.put(21, "U");
    codes.put(22, "V");
    codes.put(23, "W");
    codes.put(24, "X");
    codes.put(25, "Y");
    codes.put(26, "Z");
    }

    private static int decode(String str){

        construct();        

        int n = str.length();
        int []count = new int[n+1];

        count[0] = 1;
        count[1] = 1;

            for (int i =2; i<=n; i++){
            // If you have 0's, then they don't have a corresponding singular letter. Break off the recursion.
            if (i+1 <=n)    
             if (str.substring(i, i+1).equals("0"))
              continue;

            String x = null;
            
            if (i+1 <=n)
                x = codes.get(Integer.parseInt(str.substring(i, i+1)));
            
            if (x != null)
                count[i] = count[i-1];
                
                // If it's more than 26, it doesn't have a corresponding letter. Break off the recursion.
                if (i+2 <=n)
                 if (Integer.parseInt(str.substring(i, i+2)) <= 26)
                  count[i] += count[i-2];
                
            }

            return count[n];

        }
    
    public static void main(String[] args) {
        System.out.println(decode(args[0]));
    }
}