lc 463.easy 岛屿的周长（二维数组，数学问题）

我看的代码 https://leetcode-cn.com/problems/island-perimeter/comments/43681

官方题解 https://leetcode-cn.com/problems/island-perimeter/solution/dao-yu-de-zhou-chang-by-leetcode-solution/

class Solution {
public:
    int islandPerimeter(vector<vector<int>>& grid) {
        int len=grid.size(), c=0, len1=0;
        if(len==0) return 0;
        for(int i=0;i<len;++i){
            len1=grid[i].size();
            for(int j=0; j<len1; ++j){
                if(grid[i][j]==1){
                    c+=4;
                    if(j>0 && grid[i][j-1]==1) c-=2;  //注意这两个if包含前面的if内
                    if(i>0 && grid[i-1][j]==1) c-=2;
                }
            }
        }
        return c;
    }
};