LeetCode 892 - three-dimensional body surface area

Description Title
on N * N grid, we placed several cubes 1 * 1 * 1.
Each value v = grid [i] [j ] denotes the v cube superposed on the corresponding cell (i, j).
Please return to the final body surface area.
Example 1:
Input: [[2]]
Output: 10
Example 2:
Input: [[1,2], [3,4]]
Output: 34
Example 3:
Input: [[1,0], [0,2 ]]
output: 16
example 4:
input: [[1,1,1], [1,0,1], [1,1,1]]
output: 32
example 5:
input: [[2,2,2 ], [2,1,2], [2,2,2]]
output: 46

Solution: violent solution

In Example 2, for example,

First, a column to see a pillar, each pillar is composed of: two bottom surface (upper surface / lower surface) of the cube have contributed + all four side surface area

Then, after the column thoughtful together, we need to be bonded to lose surface area, the surface area of ​​two columns that generally conform to the minimum of two high pillars 2 *

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