3D_surface_area.py

# source: https://www.hackerrank.com/challenges/3d-surface-area/problem
# video: https://youtu.be/tIG0GF2EqjE

def surfaceArea(A):
    n_rows = len(A)
    n_cols = len(A[0])
    total = (n_rows * n_cols) * 2 # top and bottom 
    for row_i in range(n_rows): # O(n)
        for col_i in range(n_cols): # O(m)
            val = A[row_i][col_i]

            # handle row
            if row_i - 1 < 0:
                prev_row_val = 0
            else:
                prev_row_val = A[row_i - 1][col_i]

            total += abs(val - prev_row_val)

            # handle col
            if col_i - 1 < 0:
                prev_col_val = 0
            else: 
                prev_col_val = A[row_i][col_i - 1]
            total += abs(val - prev_col_val)

            # add last col element again            
            if col_i == n_cols - 1:
                total += val

        # add last row element again
        if row_i == n_rows - 1:
            total += sum(A[row_i])

    return total # Total Complexity = O(n * m)