4. 岛屿的最大面积（中等）

地址: https://leetcode-cn.com/problems/max-area-of-island/
2022/01/27
做题反思：

class Solution {
    public int maxAreaOfIsland(int[][] grid) {
        int res = 0;
        int m = grid.length, n = grid[0].length;
        for (int i = 0; i < m; i++) {
            for (int j = 0; j < n; j++) {
                if (grid[i][j] == 1) {
                    res = Math.max(res, dfs(grid, i, j));
                }
            }
        }
        return res;
    }

    int dfs(int[][] grid, int i, int j) {
        if (i < 0 || j < 0 || i >= grid.length || j >= grid[0].length) {
            return 0;
        }
        if (grid[i][j] == 0) {
            return 0;
        }
        grid[i][j] = 0;
        return dfs(grid, i, j - 1) +
            dfs(grid, i, j + 1) + 
            dfs(grid, i - 1, j) +
            dfs(grid, i + 1, j) + 1;
    }
}

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5. 统计子岛屿（中等）

地址: https://leetcode-cn.com/problems/count-sub-islands/
2022/01/27
做题反思：

class Solution {
    public int countSubIslands(int[][] grid1, int[][] grid2) {
        int res = 0;
        int m = grid1.length, n = grid1[0].length;
        for (int i = 0; i < m; i++) {
            for (int j = 0; j < n; j++) {
                if (grid1[i][j] == 0 && grid2[i][j] == 1) {
                    dfs(grid2, i, j);
                }
            }
        }
        for (int i = 0; i < m; i++) {
            for (int j = 0; j < n; j++) {
                if (grid2[i][j] == 1) {
                    res++;
                    dfs(grid2, i, j);
                }
            }
        }
        return res;
    }

    void dfs(int[][] grid, int i, int j) {
        if (i < 0 || j < 0 || i >= grid.length || j >= grid[0].length) {
            return;
        }
        if (grid[i][j] == 0) {
            return;
        }
        grid[i][j] = 0;
        dfs(grid, i, j + 1);
        dfs(grid, i, j - 1);
        dfs(grid, i - 1, j);
        dfs(grid, i + 1, j);
    }
}