分析：主要注意v = 0時引起周邊表面積的變化

class Solution {
public:
    int surfaceArea(vector<vector<int>>& grid) {
        int area = 0;
        //up/down
        for(int i = 0; i < grid.size(); i++) {
           for(int j = 0; j < grid[0].size(); j++) {
                if(grid[i][j] > 0) area += 2; //v為0的沒有表面積
           }
        }
        // cout << area << '\n';
        //left/right
        for(int j = 0; j < grid[0].size(); j++) {
            for(int i = 0; i < grid.size(); i++) {
                if(grid[i][j]) {
                    if(i == 0 || grid[i-1][j] == 0)  //最左邊或左邊為空
                        area += grid[i][j];
                    else area += abs(grid[i-1][j]-grid[i][j]); //相鄰面積之差
                }
                else if(i && grid[i-1][j])  //當前為0但前左邊一個不為0
                    area += grid[i-1][j];
            }
            if(grid[grid.size()-1][j]) area += grid[grid.size()-1][j]; //最右邊的側面
        }
        // cout << area << '\n'; 
        //front/back
        for(int i = 0; i < grid.size(); i++) {
            for(int j = 0; j < grid[0].size(); j++) {
                if(grid[i][j]) {
                    if(j == 0 || grid[i][j-1] == 0)  //最前邊或前邊為空
                        area += grid[i][j];
                    else area += abs(grid[i][j-1]-grid[i][j]); //相鄰面積之差
                }
                else if(j && grid[i][j-1])  //當前為0但前一個不為0
                    area += grid[i][j-1];
            }
            if(grid[i][grid[0].size()-1]) area += grid[i][grid[0].size()-1]; //最後邊的側面
        }
        // cout << area << '\n'; 
        return area;
    }
};