each cell potentially contribute 4 to total perimeter, if the cell is connected by another one, then we should rule out the 1 connected border because this border is not counted as outside border now

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