892. Surface Area of 3D Shapes

// Approach: Per cell contribute 4·h+2 faces; subtract 2·min(h, neighbor) for each shared wall with adjacent cells.
// Time: O(n²) Space: O(1)

public class Solution
{
    public int SurfaceArea(int[][] grid)
    {
        int ans = 0;
        int n = grid.Length;

        for (int i = 0; i < n; i++)
        {
            for (int j = 0; j < n; j++)
            {
                if (grid[i][j] > 0)
                    ans += grid[i][j] * 4 + 2;

                if (i > 0)
                    ans -= Math.Min(grid[i][j], grid[i - 1][j]) * 2;

                if (j > 0)
                    ans -= Math.Min(grid[i][j], grid[i][j - 1]) * 2;
            }
        }

        return ans;
    }
}