An inversion in a list is a pair of positions (i, j) where i comes before j but the value at i is greater than the value at j. Write a method that counts the total number of inversions in a list of integers. Return 0 for a null list or a list with fewer than two elements.
Examples:
Input: [1, 2, 3]
Output: 0
Explanation: The list is already sorted, so there are no inversions
Input: [3, 2, 1]
Output: 3
Explanation: The pairs (3,2), (3,1), and (2,1) are all inversions
Input: [2, 1, 3, 1]
Output: 3
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