463. Island Perimeter

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