[leetcode]200. Number of Islands岛屿个数

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.

Input:
11110
11010
11000
00000

Output: 1

Input:
11000
11000
00100
00011

Output: 3

思路：

- DFS and flip the bit-1 on the grid to 0 as we go: to 4 different directions
- Loop through all positions
- Visited spots won't be visited again because they are updated to '0'

怎样check 当前这格是否被visited过？

1.开一个boolean[][]  sized as the original input matrix.  但需要额外空间

2.directly flag ‘2’ or ‘0’ (any char excpet ‘1’) in the original input matrix  插个小旗flag一下

相关小岛题变种

[leetcode]694. Number of Distinct Islands你究竟有几个异小岛？

代码：

class Solution {
     public int numIslands(char[][] grid) {
        if ( (grid.length==0) || (grid[0].length==0) ) return 0;
        int count =0;
        for (int i = 0; i<grid.length;i++) {
            for (int j=0;j<grid[i].length;j++) {
                if (grid[i][j]=='1') {
                    count++;
                    helper(grid, i, j);
                }
            }
        }

        return count;
    }

    private void helper(char[][] grid, int i, int j) {
        if ((i<0) || (j<0) || (i>=grid.length) || (j>=grid[0].length)
           || grid[i][j] != '1' ) return;

        grid[i][j] = '0'; // mark visited spot
        helper(grid, i+1, j);
        helper(grid, i-1, j);
        helper(grid, i, j+1);
        helper(grid, i, j-1);
    }
}