Number of Closed Islands

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array · depth-first search · breadth-first search · union-find · matrix

Description

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

Solution