463. Island Perimeter | zhengxingxing

Method 1

You are given row x col grid representing a map where grid[i][j] = 1 represents land and grid[i][j] = 0 represents water.
Grid cells are connected horizontally/vertically (not diagonally). The grid is completely surrounded by water, and there is exactly one island (i.e., one or more connected land cells).
The island doesn’t have “lakes”, meaning the water inside isn’t connected to the water around the island. One cell is a square with side length 1. The grid is rectangular, width and height don’t exceed 100. Determine the perimeter of the island.

Example 1

Input: grid = [ [ 0,1,0,0],[1,1,1,0],[0,1,0,0],[1,1,0,0 ] ]
Output: 16
Explanation: The perimeter is the 16 yellow stripes in the image above.

Example 2

Input: grid = [ [ 1 ] ]
Output: 4

Example 3

Input: grid = [ [ 1,0 ] ]
Output: 4

Method 1

【O(m * n) time | O(1) space】

package Leetcode.Graphs;

/**
 * @Author zhengxingxing
 * @Date 2025/06/27
 */
public class IslandPerimeter {

    public static int islandPerimeter(int[][] grid) {
        int m = grid.length;        // Number of rows in the grid
        int n = grid[0].length;     // Number of columns in the grid
        int perimeter = 0;          // Variable to store the total perimeter

        // Traverse each cell in the grid
        for (int i = 0; i < m; i++) {
            for (int j = 0; j < n; j++) {
                // If the current cell is land
                if(grid[i][j] == 1){
                    // Each land cell initially contributes 4 to the perimeter
                    perimeter += 4;

                    // If the cell above (i-1, j) is also land,
                    // it means the current cell shares a border with the upper cell.
                    // Each shared border should be subtracted twice (once for each cell),
                    // so we subtract 2 from the perimeter.
                    if (i > 0 && grid[i - 1][j] == 1){
                        perimeter -= 2;
                    }

                    // If the cell to the left (i, j-1) is also land,
                    // it means the current cell shares a border with the left cell.
                    // Again, subtract 2 for the shared border.
                    if (j > 0 && grid[i][j - 1] == 1) {
                        perimeter -= 2;
                    }
                }
            }
        }

        // Return the total calculated perimeter
        return perimeter;
    }

    public static void main(String[] args) {
        // Test case 1: Example from the problem description
        int[][] grid1 = {
                {0,1,0,0},
                {1,1,1,0},
                {0,1,0,0},
                {1,1,0,0}
        };
        System.out.println(islandPerimeter(grid1)); // Output: 16

        // Test case 2: Single land cell
        int[][] grid2 = {
                {1}
        };
        System.out.println(islandPerimeter(grid2)); // Output: 4

        // Test case 3: Two land cells in a row
        int[][] grid3 = {
                {1,0}
        };
        System.out.println(islandPerimeter(grid3)); // Output: 4
    }
}

Method 1

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