public static int altSum(IntNode head, int i)

which, when called as altSum(head, 1), computes the alternating sum of the elements in the IntMode-based linked-list with head-reference head, i.e. Sigma_{i=i}^{n}(sign(i) * val(i)) where n is the number of nodes in the list, val(i) is the value associated with the i-th element in the list, and sign(i) is 1 if i is odd and -1 if i is even. For example, given the list

head
[ ] --> [4][ ] --> [7][ ] --> [8][ ] --> [2][ ] --> [9][X]
the alternating sum for thos list is 4+-7+8+-2+9=12.

public static int altSum(IntNode head, int i)
{
     if (i%2==0)
     {
           int sign = -1;
     }
     else
     {
          int sign = 1;
     }

     if (head == null) //base case
     {
           return 0;
     }
     else //general case
     {
          return( (sign*list.get(i)) + (altSum(head.next, i++)) );
     }
}