public static void printout(int[] tree, int index)
   {
      //if the left node is 0, print value in parent node
      if (tree[index*2] == 0) {
         System.out.println(tree[index]);

         //if the right node is 0, return to previous node
         if(tree[index*2+1] == 0) {
            System.out.println(tree[index/2]);

         //if the right node is not 0, go to next right node
         } else {
            printout(tree, index*2+1);
         }

      //if left node is not 0, go to next left node
      } else {
         printout(tree, index*2);
      }