Problem Statement
Count pairs (i,j) where i < j and arr[i] > arr[j].
Approach
Merge sort. Count inversions during merge step.
Time & Space Complexity
Time O(n log n), Space O(n).

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Modified merge sort for counting.
Count pairs (i,j) where i < j and arr[i] > arr[j].
Merge sort. Count inversions during merge step.
Time O(n log n), Space O(n).
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