Number of Closed Islands — LeetCode 1254 Python Solution

The problem

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.

Example

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).

Python solution

class Solution:
    def closedIsland(self, grid: List[List[int]]) -> int:
        def dfs(i: int, j: int) -> int:
            res = int(0 < i < m - 1 and 0 < j < n - 1)
            grid[i][j] = 1
            for a, b in pairwise(dirs):
                x, y = i + a, j + b
                if 0 <= x < m and 0 <= y < n and grid[x][y] == 0:
                    res &= dfs(x, y)
            return res

        m, n = len(grid), len(grid[0])
        dirs = (-1, 0, 1, 0, -1)
        return sum(grid[i][j] == 0 and dfs(i, j) for i in range(m) for j in range(n))

Complexity

Pattern: Union-Find

Merge groups and ask whether two things are connected, both in near-constant time. LeetCode 1254. Number of Closed Islands is filed here because LeetCode tags it Union Find, which is the vocabulary this hub collects.

The union-find guide has the Python template for the pattern and the 83 LeetCode problems that use it.

Related problems