Write a method that computes the alternating sum of a list of integers: add the first element, subtract the second, add the third, subtract the fourth, and so on. Return 0 for a null or empty list.
Examples:
Input: [1, 2, 3, 4]
Output: -2
Explanation: 1 - 2 + 3 - 4 = -2
Input: [10, 3]
Output: 7
Input: [5]
Output: 5
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