LeetCode-in-Java

1254. Number of Closed Islands

Medium

Given a 2D grid consists of 0s (land) and 1s (water). An island is a maximal 4-directionally connected group of 0s and a closed island is an island totally (all left, top, right, bottom) surrounded by 1s.

Return the number of closed islands.

Example 1:

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

Output: 2

Explanation: Islands in gray are closed because they are completely surrounded by water (group of 1s).

Example 2:

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

Output: 1

Example 3:

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

Output: 2

Constraints:

• 1 <= grid.length, grid[0].length <= 100
• 0 <= grid[i][j] <=1

Solution

public class Solution {
    private int rows;
    private int cols;
    private boolean isLand;

    public int closedIsland(int[][] grid) {
        rows = grid.length;
        cols = grid[0].length;

        int result = 0;
        for (int r = 0; r < rows; r++) {
            for (int c = 0; c < cols; c++) {
                if (grid[r][c] == 0) {
                    isLand = true;
                    dfs(grid, r, c);
                    if (isLand) {
                        result++;
                    }
                }
            }
        }
        return result;
    }

    private void dfs(int[][] grid, int r, int c) {
        if (r == 0 || c == 0 || r == rows - 1 || c == cols - 1) {
            isLand = false;
        }
        grid[r][c] = 'k';
        if (r > 0 && grid[r - 1][c] == 0) {
            dfs(grid, r - 1, c);
        }
        if (c > 0 && grid[r][c - 1] == 0) {
            dfs(grid, r, c - 1);
        }
        if (r < rows - 1 && grid[r + 1][c] == 0) {
            dfs(grid, r + 1, c);
        }
        if (c < cols - 1 && grid[r][c + 1] == 0) {
            dfs(grid, r, c + 1);
        }
    }
}

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