3D surface

    // Complete the surfaceArea function below.
    static int surfaceArea(int[][] A) {
        int surface = A.length * A[0].length * 2;
        for (int i = 0; i < A.length; i++) {
            for (int j = 0; j < A[i].length; j++) {
                // check top
                int topNeighbour = i == 0 ? 0 : A[i-1][j];
                int diff = A[i][j] - topNeighbour;
                if (diff > 0) {
                    surface += diff;
                }

                // check bottom
                int bottomNeighbour = i == A.length - 1 ? 0 : A[i + 1][j];
                diff = A[i][j] - bottomNeighbour;
                if (diff > 0) {
                    surface += diff;
                }

                // check left
                int leftNeighbour = j == 0 ? 0 : A[i][j - 1];
                diff = A[i][j] - leftNeighbour;
                if (diff > 0) {
                    surface += diff;
                }

                // check right
                int rightNeighbour = j == A[i].length - 1 ? 0 : A[i][j + 1];
                diff = A[i][j] - rightNeighbour;
                if (diff > 0) {
                    surface += diff;
                }
            }
        }
        return surface;
    }