LeetCode 463 Island Perimeter - Core Idea

Here is the approach:

$1.$ Loop through every cell in the grid.

$2.$ If the cell is land ($1$), start with $4$ edges.

$3.$ Check the $4$ neighbors (up, down, left, right). For each neighbor that is also land, subtract $1$ from the edge count.

$4.$ Add the remaining edges to the total perimeter.

$5.$ Return the total after visiting all cells.

This works because each land neighbor "hides" one edge. If a cell has $3$ land neighbors, only $1$ edge is exposed. The simplicity comes from the fact that there is only one island, so you do not need to track visited cells.