LeetCode 883: Projection Area of 3D Shapes

Meta Data

#

Difficulty: easy First Attempt: 2022-09-21

• Submissions: Runtime Error ×4 → Wrong Answer ×2 → Accepted

Approach

#

The three projections (Matrix Template 3):

• From above: each cell with grid[i][j] > 0 contributes 1.
• From the front: the tallest stack in each column.
• From the side: the tallest stack in each row.

My Attempts

#

The first attempts were variable and type REs. The next two joined every number into one string and counted '0' characters to subtract empty cells, but once any cell is empty, the '0' inside numbers such as 10 and 20 is counted too: [[0, 10]] should be 21; it returned 20. Testing grid[i][j] > 0 passed.

Solution

#

class Solution:
    def projectionArea(self, grid: List[List[int]]) -> int:
        top = sum(v > 0 for row in grid for v in row)
        side = sum(max(row) for row in grid)
        front = sum(max(col) for col in zip(*grid))
        return top + side + front

Complexity

#

Time O(n²), space O(1).

Mistakes to Avoid

#

• The top view counts nonzero cells; do not count '0' characters in a string.

Interview Explanation

#

The top view is the number of nonzero cells, and the front and side views sum the maxima of the columns and rows; I add all three. Time O(n²).