Surface Area of 3D Shapes

<-E 892> Surface Area of 3D Shapes

class Solution {
public:
    int surfaceArea(vector<vector<int>>& grid) {
        int n = grid.size(), res = 0;
        for (int i = 0; i < n; ++i) {
            for (int j = 0; j < n; ++j) {
                if (grid[i][j] > 0) res += 4 * grid[i][j] + 2;
                if (i > 0) res -= min(grid[i][j], grid[i - 1][j]) * 2;
                if (j > 0) res -= min(grid[i][j], grid[i][j - 1]) * 2;
            }
        }
        return res;
    }
};