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:

11110
11010
11000
00000

Answer: 1

Example 2:

11000
11000
00100
00011

Answer: 3

120/145

思路：跟word serach很像。对matrix上的每一个点进行dfs，dfs完一次就把这次搜索到的岛屿上所有点都转成0，这样下次搜索就不会搜索到重复的点。注意这种matrix搜索的base条件都是i>grid.length-1||j>grid[0].length-1||i<0||j<0。

public class Solution {
    public int numIslands(char[][] grid) {
        if(grid==null||grid.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')
                {
                    dfs(grid,i,j);
                    ++count;
                }
            }
        }
    
    return count;
    }
    
    private void dfs(char[][] grid, int i, int j)
    {
        if(i>grid.length-1||j>grid[0].length-1||i<0||j<0||grid[i][j]!='1')
        {
            return;
        }
        grid[i][j]='0';
        
        dfs(grid,i-1,j);
        dfs(grid,i,j-1);
        dfs(grid,i+1,j);
        dfs(grid,i,j+1);
        
        
    }
    
}