100道面试必会算法-18-岛屿问题（数量、周长、面积）

题目描述

给你一个由 '1'（陆地）和 '0'（水）组成的的二维网格，请你计算网格中岛屿的数量。

岛屿总是被水包围，并且每座岛屿只能由水平方向和/或竖直方向上相邻的陆地连接形成。

此外，你可以假设该网格的四条边均被水包围。

示例 1：

输入：grid = [
  ["1","1","1","1","0"],
  ["1","1","0","1","0"],
  ["1","1","0","0","0"],
  ["0","0","0","0","0"]
]
输出：1

示例 2：

输入：grid = [
  ["1","1","0","0","0"],
  ["1","1","0","0","0"],
  ["0","0","1","0","0"],
  ["0","0","0","1","1"]
]
输出：3

解题思路

采用深度优先遍历思想，传送门：https://leetcode.cn/problems/number-of-islands/solutions/211211/dao-yu-lei-wen-ti-de-tong-yong-jie-fa-dfs-bian-li-/

值得注意一个问题

以上写法问题，if内判断是需要注意顺序的，倘若发生越界要注意判断条件，grid[i][j]!= '1'的判断需要在i<0,j<0后，若先进行判断则可能会发生越界错误

代码

public class LC13 {
    public int numIslands(char[][] grid) {
        int count=0;
        for (int i = 0; i < grid.length; i++) {
            for (int j = 0; j < grid[0].length; j++) {
                if (grid[i][j]=='1'){
                    dfs(grid,i,j);
                    count++;
                }
            }
        }
        return count;
    }

    private void dfs(char[][] grid, int i, int j) {
//        第一种写法（不通过）
//        if (i >= grid.length || j >= grid[0].length || grid[i][j] != '1' || i<0|| j<0)return;

//        第二种写法（通过）
          if (i<0 || j<0 || i >= grid.length || j >= grid[0].length || grid[i][j] != '1' )return;

//        第三种写法（通过）
//        if (i >= grid.length || j >= grid[0].length || i < 0 || j < 0) {
//            return;
//        }
//        if (grid[i][j] != '1') {
//            return;
//        }
        grid[i][j]='2';
        dfs(grid, i-1, j);
        dfs(grid, i+1, j);
        dfs(grid, i, j-1);
        dfs(grid, i, j+1);
    }
    public static void main(String[] args) {
        // 创建一个示例对象
        LC13 main = new LC13();
        // 测试用例
        char[][] grid = {
                {'1', '1', '0', '0', '0'},
                {'1', '1', '0', '0', '0'},
                {'0', '0', '1', '0', '0'},
                {'0', '0', '0', '1', '1'}
        };
        // 调用numIslands方法计算岛屿数量
        int islands = main.numIslands(grid);
        // 输出结果
        System.out.println("岛屿数量为：" + islands);
    }
}

面积

int area(int[][] grid, int r, int c) {  
    return 1 
        + area(grid, r - 1, c)
        + area(grid, r + 1, c)
        + area(grid, r, c - 1)
        + area(grid, r, c + 1);
}

周长

c)
+ area(grid, r + 1, c)
+ area(grid, r, c - 1)
+ area(grid, r, c + 1);
}

周长