[leedcode 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:

11110110101100000000

Answer: 1

Example 2:

11000110000010000011

Answer: 3

public class Solution {
    public int numIslands(char[][] grid) {
        //dfs思想，当发现一个节点是1时，需要将跟其相邻的所有节点置为0，此种方法会改变原grid的值
        if(grid==null||grid.length<=0) return 0;
        int row=grid.length;
        int col=grid[0].length;
        int count=0;
        for(int i=0;i<row;i++){
            for(int j=0;j<col;j++){
                if(grid[i][j]==‘1‘){
                    count++;
                    dfs(grid,i,j);
                }
            }
        }
        return count;
    }
    public void dfs(char[][] grid,int i,int j){
        if(i<0||i>grid.length-1||j<0||j>grid[0].length-1) return;
        if(grid[i][j]==‘1‘){
            grid[i][j]=‘0‘;
            dfs(grid,i-1,j);
            dfs(grid,i+1,j);
            dfs(grid,i,j-1);
            dfs(grid,i,j+1);
        }
    }
}