Count Inversions in an Array?

Short answer: int MergeSortAndCount(int[] arr, int[] temp, int left, int right) { int invCount = 0; if (right > left) { int mid = (right + left) / 2; invCount += MergeSortAndCount(arr, temp, left, mid); invCount += MergeSortAndCount(arr, temp, mid + 1, right); invCount += Merge(arr, temp, left, mid + 1, right); } return invCount; } int Merge(int[] arr, int[] temp, int left, int mid, int right) { int i = left, j = mid, k = left;…

Explain a bit more

int invCount = 0; while (i <= mid - 1 && j <= right) { if (arr[i] <= arr[j]) temp[k++] = arr[i++]; else { temp[k++] = arr[j++]; invCount += (mid - i); // Count inversions } } while (i <= mid - 1) temp[k++] = arr[i++]; Follow on: while (j <= right) temp[k++] = arr[j++]; for (int idx = left; idx <= right; idx++) arr[idx] = temp[idx]; return invCount; } Explanation: Using a modified merge sort to count pairs where arr[i] > arr[j] for i < j efficiently.

Example code

int MergeSortAndCount(int[] arr, int[] temp, int left, int right)
{
int invCount = 0;
if (right > left)
{
int mid = (right + left) / 2;
invCount += MergeSortAndCount(arr, temp, left, mid);
invCount += MergeSortAndCount(arr, temp, mid + 1, right);
invCount += Merge(arr, temp, left, mid + 1, right);
}
return invCount;
}
int Merge(int[] arr, int[] temp, int left, int mid, int right)
{
int i = left, j = mid, k = left;
int invCount = 0; while (i <= mid - 1 && j <= right) {
if (arr[i] <= arr[j])
temp[k++] = arr[i++]; else {
temp[k++] = arr[j++];
invCount += (mid - i); // Count inversions
}
} while (i <= mid - 1) temp[k++] = arr[i++]; Follow on: while (j <= right) temp[k++] = arr[j++];
for (int idx = left; idx <= right; idx++)
arr[idx] = temp[idx];
return invCount;
} Explanation: Using a modified merge sort to count pairs where arr[i] > arr[j] for i < j efficiently.

Real-world example (ShopNest)

In coding rounds, state complexity aloud, write a clear ShopNest-flavored example (orders, carts), then handle edge cases (empty list, null, overflow).

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.