1254. Number of Closed Islands

题目

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

题目大意

有一个二维矩阵 grid ，每个位置要么是陆地（记号为 0 ）要么是水域（记号为 1 ）。我们从一块陆地出发，每次可以往上下左右 4 个方向相邻区域走，能走到的所有陆地区域，我们将其称为一座「岛屿」。如果一座岛屿 完全 由水域包围，即陆地边缘上下左右所有相邻区域都是水域，那么我们将其称为 「封闭岛屿」。请返回封闭岛屿的数目。

提示：

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

解题思路

• 给出一个地图，1 代表海水，0 代表陆地。要求找出四周都是海水的陆地的总个数。
• 这一题和第 200 题解题思路完全一致。只不过这一题要求必须四周都是海水，第 200 题的陆地可以是靠着地图边缘的。在此题中，靠着地图边缘的陆地不能最终计数到结果中。

代码

package leetcode

func closedIsland(grid [][]int) int {
	m := len(grid)
	if m == 0 {
		return 0
	}
	n := len(grid[0])
	if n == 0 {
		return 0
	}
	res, visited := 0, make([][]bool, m)
	for i := 0; i < m; i++ {
		visited[i] = make([]bool, n)
	}
	for i := 0; i < m; i++ {
		for j := 0; j < n; j++ {
			isEdge := false
			if grid[i][j] == 0 && !visited[i][j] {
				checkIslands(grid, &visited, i, j, &isEdge)
				if !isEdge {
					res++
				}

			}
		}
	}
	return res
}

func checkIslands(grid [][]int, visited *[][]bool, x, y int, isEdge *bool) {
	if (x == 0 || x == len(grid)-1 || y == 0 || y == len(grid[0])-1) && grid[x][y] == 0 {
		*isEdge = true
	}
	(*visited)[x][y] = true
	for i := 0; i < 4; i++ {
		nx := x + dir[i][0]
		ny := y + dir[i][1]
		if isIntInBoard(grid, nx, ny) && !(*visited)[nx][ny] && grid[nx][ny] == 0 {
			checkIslands(grid, visited, nx, ny, isEdge)
		}
	}
	*isEdge = *isEdge || false
}

func isIntInBoard(board [][]int, x, y int) bool {
	return x >= 0 && x < len(board) && y >= 0 && y < len(board[0])
}

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