How to check if an instance of 8 puzzle is solvable

It is not possible to solve an instance of 8 puzzle if number of inversions is odd in the input state.

What is inversion?
A pair of tiles form an inversion if the the values on tiles are in reverse order of their appearance in goal state. For example, the following instance of 8 puzzle has two inversions, (8, 6) and (8, 7).

   1   2   3
   4   _   5
   8   6   7

CODE

int getInvCount(int arr[])
{
    int inv_count = 0;
    for (int i = 0; i < 9 - 1; i++)
        for (int j = i+1; j < 9; j++)
             // Value 0 is used for empty space
             if (arr[j] && arr[i] &&  arr[i] > arr[j])
                  inv_count++;
    return inv_count;
}

bool isSolvable(int puzzle[3][3])
{
    // Count inversions in given 8 puzzle
    int invCount = getInvCount((int *)puzzle);

    // return true if inversion count is even.
    return (invCount%2 == 0);
}