Given a 2d grid map of
'1'
s (land) and '0'
s (water), count the number of islands. An island is surrounded by water and is formed by connecting adjacent lands horizontally or vertically. You may assume all four edges of the grid are all surrounded by water.
Example 1:
11110 11010 11000 00000
Answer: 1
Example 2:
11000 11000 00100 00011
Answer: 3
Solution: This is to find the number of connected components. DFS or BFS is favorable.
public class Solution { public boolean isVisitable( char[][] grid, int[][] visited, int i, int j ) { return ( i >= 0 && i < grid.length && j >= 0 && j < grid[0].length && grid[i][j] == '1' && visited[i][j] == 0 ); } public void DFS(char[][] grid, int[][] visited, int i, int j) { visited[i][j] = 1; if (isVisitable(grid, visited, i-1, j)) { DFS(grid, visited, i-1, j); } if (isVisitable(grid, visited, i+1, j)) { DFS(grid, visited, i+1, j); } if (isVisitable(grid, visited, i, j-1)) { DFS(grid, visited, i, j-1); } if (isVisitable(grid, visited, i, j+1)) { DFS(grid, visited, i, j+1); } return; } public int numIslands(char[][] grid) { int m = grid.length; if (m == 0) return 0; int n = grid[0].length; int[][] visited = new int[m][n]; for (int[] row : visited) { Arrays.fill(row, 0); } int num = 0; for (int i = 0; i < m; i++) { for (int j = 0; j < n; j++) { if (grid[i][j] == '1' && visited[i][j] == 0) { num++; DFS(grid, visited, i, j); } } } return num; } }
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