My C# solution:

public static int diagonalDifference(List<List<int>> arr)
{
        var diagonalSumA = 0;
        var diagonalSumB = 0;
        var rowCounter = 0;

        foreach (var item in arr) {
            for (int i = 0; i < item.Count; i++) {
                if (rowCounter == i) {
                    diagonalSumA += item[i];
                }

                if (i == (-rowCounter + (item.Count - 1))) {
                    diagonalSumB += item[i];
                }
            }

            rowCounter++;
        }

        return Math.Abs(diagonalSumA - diagonalSumB);
}[](https://)