Solution
Approach 1: Mathematical
Intuition and Algorithm
From the top, the shadow made by the shape will be 1 square for each nonzero value.
From the side, the shadow made by the shape will be the largest value for each row in the grid.
From the front, the shadow made by the shape will be the largest value for each column in the grid.
Example
With the example [[1,2],[3,4]]
:

The shadow from the top will be 4, since there are four nonzero values in the grid;

The shadow from the side will be
2 + 4
, since the maximum value of the first row is2
, and the maximum value of the second row is4
; 
The shadow from the front will be
3 + 4
, since the maximum value of the first column is3
, and the maximum value of the second column is4
.
Complexity Analysis

Time Complexity: , where is the length of
grid
. 
Space Complexity: .
Analysis written by: @awice.