Number of Closed Islands
Difficulty: Medium
Category: DSA
Topics: Array, Depth-First Search, Breadth-First Search, Union Find, Matrix
Asked at: Google, Amazon, Oracle, Uber
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).
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**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`