Write a method that returns the sum of all the digits in 'num' except that the SIGN of each digit alternates in such a way that the ones place is positive, the tens place is negative, the hundreds place is positive, and so on. For example, if 'num' is 5718, then the alternating sum will be (-5) + 7 + (-1) + 8 = 9. Notice that the ones place is always positive, so the alternating begins with the least significant digit. Look at the test data for examples. As a precondition, you may assume that 'num' is positive.