validPhoneNumber

/*
Print all valid phone numbers of length n subject to following constraints: 

1.If a number contains a 4, it should start with 4 
2.No two consecutive digits can be same 
3.Three digits (e.g. 7,2,9) will be entirely disallowed, take as input
*/


private static void validPhoneNumber(int d, int n, int s, String result, String pattern){
  if (d == n){
		if (checkvalid(result))
			System.out.println(result);
		return;
	}
	else {
		for (int i=0; i<pattern.length(); i++){
			if (!keys.containsKey(i)) {
				//keys.put(i, true);  // controls if the same digit is allowed to show up for more than one time.
				validPhoneNumber(d+1, n, i+1, result+i, patttern); // s controls if this is combination or permutation
				//keys.remove(i);
			}
		}
	}
	return;
		
}

private static boolean checkvalid(String result){
	int count = 0;
	for (int i=1; i<result.length(); i++){
		if (result.charAt(i) == '4') count++;
		if (result.charAt(i) == result.charAt(i-1))
			return false;
	}
	
	return(count==0);

}


private static String pattern = "0134568";
private static Hashtable<Integer, Boolean> keys = new Hashtable<Integer, Boolean>();