200. Number of Islands-蒲公英云

200. Number of Islands

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:
Input:
11110
11010
11000
00000
Output: 1
Example 2:
Input:
11000
11000
00100
00011
Output: 3

本题是一个非常经典的深度优先搜索的题目（代码我都能背出来了）。代码如下：

int[] dx = new int[]{0, 1, 0, -1};
    int[] dy = new int[]{1, 0, -1, 0};
    public int numIslands(char[][] grid) {
        int ans = 0;
        for(int i = 0; i < grid.length; ++i) {
            for(int j = 0; j < grid[0].length; ++j) {
                if(grid[i][j] == '1') {
                    ++ans;
                    grid[i][j] = '0';
                    dfs(i, j, grid);
                }
            }
        }
        return ans;
    }
    private void dfs(int i, int j, char[][] grid) {
        for(int k = 0; k < 4; ++k) {
            int x = i + dx[k];
            int y = j + dy[k];
            if(0 <= x && x < grid.length && 0 <= y && y < grid[0].length && grid[x][y] == '1') {
                grid[x][y] = '0';
                dfs(x, y, grid);
            }
        }
    }