Number of Islands——LeetCode进阶路

原题链接https://leetcode.com/problems/number-of-islands/

题目描述

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

思路分析

这道题和上一题 “围绕的区域”https://blog.csdn.net/Moliay/article/details/88584035

思路相似，同用dfs，特定元素访问之后标记，递归出结果 bingo

源码附录

class Solution {
    public int numIslands(char[][] grid) {
        if(grid == null || grid.length == 0 || grid[0].length == 0)
        {
            return 0;
        }

        int row = grid.length;
        int col = grid[0].length;
        int result = 0;

        for(int i=0;i<row;i++)
        {
            for(int j=0;j<col;j++)
            {
                if(grid[i][j] == '1')
                {
                    result ++;
                    dfs(grid,i,j,row,col);
                }
            }
        }
        return result;
    }

    public void dfs(char[][] grid,int i,int j,int row,int col)
    {
        if(i>row-1 || j>col-1 || i<0 || j<0)
        {
            return;
        }  

        grid[i][j] = '2';

        if(j<col-1 && grid[i][j+1] == '1')
        {
            dfs(grid,i,j+1,row,col);
        }
        if(j>0 && grid[i][j-1] == '1')
        {
            dfs(grid,i,j-1,row,col);
        }
        if(i<row-1 && grid[i+1][j] == '1')
        {
            dfs(grid,i+1,j,row,col);
        }
        if(i>0 && grid[i-1][j] == '1')
        {
            dfs(grid,i-1,j,row,col);
        }

    }
}