LeetCode [1254] Number of Closed Islands

 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

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class Solution { int m, n, count = 0; int[][] dir = new int[][]{{-1,0},{1,0},{0,-1},{0,1}}; public int closedIsland(int[][] grid) { m = grid.length; if(m<=2) return 0; n = grid[0].length; if(n<=2) return 0; for(int i=0; i<m; ++i){ for(int j=0; j<n; ++j){ if(grid[i][j]==0 && isClosed(grid, i, j)){ count++; } } } return count; } //dfs on lands and mark visited lands by 2 boolean isClosed(int[][] grid, int i, int j){ grid[i][j] = 2; boolean closed = true; if(i==0 || i==m-1 || j==0 || j==n-1) closed = false; for(int[] d : dir){ int ii = i+d[0], jj = j+d[1]; if(ii>=0 && ii<m && jj>=0 && jj<n && grid[ii][jj]==0){ if(!isClosed(grid, ii, jj)) closed = false; } } return closed; } }

YMSF

第二轮： 蠡口易而无私的变种，最后求岛的最大面积。这一题之前没见到过。想了很久也没想出来最优解。估计表现的不好所以加面。