public class RecursiveProgramming {

   public static void main(String[] args) {
      int[] arr = {1, 2, 3};
      System.out.println(sum(arr));
   }
   
   // post: returns the sum of all the integers in the array
   public static int sum(int[] arr) {
      return sum(arr, 0);
   }
   
   // post: returns the sum of all the integers in the array
   // starting from the given index
   private static int sum(int[] arr, int index) {
      if (index == arr.length) {
         return 0;
      } else {
         int sum = arr[index] + sum(arr, index);
         return sum;
      }
   }
   
   // post: returns an integer where every digit of n
   // is replaced by two of that digit
   // Example: doubleUp(348) returns 334488
   // Example: doubleUp(-348) returns -334488
   public static int doubleUp(int n){
      if (n < 0) {
         return -doubleUp(-n);
      } else if (n < 10)  {
         return n * 11;
      } else {
         return 100 * doubleUp(n / 10) + doubleUp(n % 10);
      }
   }
   
   // post: returns a string where every character of str 
   // is replaced by two of that character
   // Example: doubleUp("cat") returns "ccaatt"
   // Example: doubleUp("") returns ""
   public static String doubleUp(String str) {
      if (str.length() <= 1) {
         return str + str;
      } else {
         char c = str.charAt(0);
         return "" + c + c + doubleUp(str.substring(1));
      }
   }
   
}