463. Island Perimeter

Math

class Solution {
    private static final int[][] DIRECTIONS = new int[][]{{1, 0}, {-1, 0}, {0, 1}, {0, -1}};

    public int islandPerimeter(int[][] grid) {
        int m = grid.length, n = grid[0].length;
        int length = 0;

        for (int i = 0; i < m; i++) {
            for (int j = 0; j < n; j++) {
                if (grid[i][j] == 1) {
                    length += 4;
                    for (int[] direction : DIRECTIONS) {
                        int nextI = i + direction[0];
                        int nextJ = j + direction[1];
                        if (nextI >= 0 && nextI < m && nextJ >= 0 && nextJ < n && grid[nextI][nextJ] == 1) {
                            length -= 1;
                        }
                    }
                }
            }
        }

        return length;
    }
}

DFS

class Solution {
    public int islandPerimeter(int[][] grid) {
        int m = grid.length, n = grid[0].length;

        for (int i = 0; i < m; i++) {
            for (int j = 0; j < n; j++) {
                if (grid[i][j] == 1) {
                    return dfs(grid, m, n, i, j);
                }
            }
        }

        return 0;
    }

    private int dfs(int[][] grid, int m, int n, int i, int j) {
        if (i < 0 || i >= m || j < 0 || j >= n || grid[i][j] == 0) {
            return 1;
        }
        
        if (grid[i][j] == 2) {
            return 0;
        }

        grid[i][j] = 2;

        return dfs(grid, m, n, i, j + 1) + dfs(grid, m, n, i, j - 1) + dfs(grid, m, n, i + 1, j) + dfs(grid, m, n, i - 1, j);
    }
}

Reference

463. Island Perimeter