代码随想录算法训练营第52天 | 101.孤岛的总面积 、102.沉没孤岛、103.水流问题、104.建造最大岛屿

 101.孤岛的总面积

import java.util.*;

public class Main {
    private static int count = 0;
    private static final int[][] dir = {{0, 1}, {1, 0}, {-1, 0}, {0, -1}}; // 四个方向

    private static void bfs(int[][] grid, int x, int y) {
        Queue que = new LinkedList<>();
        que.add(new int[]{x, y});
        grid[x][y] = 0; // 只要加入队列，立刻标记
        count++;
        while (!que.isEmpty()) {
            int[] cur = que.poll();
            int curx = cur[0];
            int cury = cur[1];
            for (int i = 0; i < 4; i++) {
                int nextx = curx + dir[i][0];
                int nexty = cury + dir[i][1];
                if (nextx < 0 || nextx >= grid.length || nexty < 0 || nexty >= grid[0].length) continue; // 越界了，直接跳过
                if (grid[nextx][nexty] == 1) {
                    que.add(new int[]{nextx, nexty});
                    count++;
                    grid[nextx][nexty] = 0; // 只要加入队列立刻标记
                }
            }
        }
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();
        int m = scanner.nextInt();
        int[][] grid = new int[n][m];
        
        // 读取网格
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < m; j++) {
                grid[i][j] = scanner.nextInt();
            }
        }
        
        // 从左侧边，和右侧边向中间遍历
        for (int i = 0; i < n; i++) {
            if (grid[i][0] == 1) bfs(grid, i, 0);
            if (grid[i][m - 1] == 1) bfs(grid, i, m - 1);
        }
        
        // 从上边和下边向中间遍历
        for (int j = 0; j < m; j++) {
            if (grid[0][j] == 1) bfs(grid, 0, j);
            if (grid[n - 1][j] == 1) bfs(grid, n - 1, j);
        }
        
        count = 0;
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < m; j++) {
                if (grid[i][j] == 1) bfs(grid, i, j);
            }
        }

        System.out.println(count);
    }
}

102.沉没孤岛 

import java.util.Scanner;

public class Main {
    static int[][] dir = { {-1, 0}, {0, -1}, {1, 0}, {0, 1} }; // 保存四个方向

    public static void dfs(int[][] grid, int x, int y) {
        grid[x][y] = 2;
        for (int[] d : dir) {
            int nextX = x + d[0];
            int nextY = y + d[1];
            // 超过边界
            if (nextX < 0 || nextX >= grid.length || nextY < 0 || nextY >= grid[0].length) continue;
            // 不符合条件，不继续遍历
            if (grid[nextX][nextY] == 0 || grid[nextX][nextY] == 2) continue;
            dfs(grid, nextX, nextY);
        }
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();
        int m = scanner.nextInt();
        int[][] grid = new int[n][m];

        for (int i = 0; i < n; i++) {
            for (int j = 0; j < m; j++) {
                grid[i][j] = scanner.nextInt();
            }
        }

        // 步骤一：
        // 从左侧边，和右侧边 向中间遍历
        for (int i = 0; i < n; i++) {
            if (grid[i][0] == 1) dfs(grid, i, 0);
            if (grid[i][m - 1] == 1) dfs(grid, i, m - 1);
        }

        // 从上边和下边 向中间遍历
        for (int j = 0; j < m; j++) {
            if (grid[0][j] == 1) dfs(grid, 0, j);
            if (grid[n - 1][j] == 1) dfs(grid, n - 1, j);
        }

        // 步骤二、步骤三
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < m; j++) {
                if (grid[i][j] == 1) grid[i][j] = 0;
                if (grid[i][j] == 2) grid[i][j] = 1;
            }
        }

        for (int i = 0; i < n; i++) {
            for (int j = 0; j < m; j++) {
                System.out.print(grid[i][j] + " ");
            }
            System.out.println();
        }
        
        scanner.close();
    }
}

 103.水流问题

public class Main {

    // 采用 DFS 进行搜索
    public static void dfs(int[][] heights, int x, int y, boolean[][] visited, int preH) {
        // 遇到边界或者访问过的点，直接返回
        if (x < 0 || x >= heights.length || y < 0 || y >= heights[0].length || visited[x][y]) return;
        // 不满足水流入条件的直接返回
        if (heights[x][y] < preH) return;
        // 满足条件，设置为true，表示可以从边界到达此位置
        visited[x][y] = true;

        // 向下一层继续搜索
        dfs(heights, x + 1, y, visited, heights[x][y]);
        dfs(heights, x - 1, y, visited, heights[x][y]);
        dfs(heights, x, y + 1, visited, heights[x][y]);
        dfs(heights, x, y - 1, visited, heights[x][y]);
    }

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int m = sc.nextInt();
        int n = sc.nextInt();

        int[][] heights = new int[m][n];
        for (int i = 0; i < m; i++) {
            for (int j = 0; j < n; j++) {
                heights[i][j] = sc.nextInt();
            }
        }

        // 初始化两个二位boolean数组，代表两个边界
        boolean[][] pacific = new boolean[m][n];
        boolean[][] atlantic = new boolean[m][n];

        // 从左右边界出发进行DFS
        for (int i = 0; i < m; i++) {
            dfs(heights, i, 0, pacific, Integer.MIN_VALUE);
            dfs(heights, i, n - 1, atlantic, Integer.MIN_VALUE);
        }

        // 从上下边界出发进行DFS
        for (int j = 0; j < n; j++) {
            dfs(heights, 0, j, pacific, Integer.MIN_VALUE);
            dfs(heights, m - 1, j, atlantic, Integer.MIN_VALUE);
        }

        // 当两个边界二维数组在某个位置都为true时，符合题目要求
        List> res = new ArrayList<>();
        for (int i = 0; i < m; i++) {
            for (int j = 0; j < n; j++) {
                if (pacific[i][j] && atlantic[i][j]) {
                    res.add(Arrays.asList(i, j));
                }
            }
        }

        // 打印结果
        for (List list : res) {
            for (int k = 0; k < list.size(); k++) {
                if (k == 0) {
                    System.out.print(list.get(k) + " ");
                } else {
                    System.out.print(list.get(k));
                }
            }
            System.out.println();
        }
    }
}