算法学习之floodfill算法

题目要求：

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
提示：

m == grid.length
n == grid[i].length
1 <= m, n <= 300
grid[i][j] 的值为 '0' 或 '1'

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/number-of-islands

解题代码：

class Solution {
private:
    int d[4][2] = {{0,  1},{1,  0},{0,  -1},{-1, 0}};
    int m, n;
    vector<vector<bool>> visited;

    bool inArea(int x, int y) {
        return x >= 0 && x < m && y >= 0 && y < n;
    }

    // 从grid[x][y]的位置开始，进行floodfill
    // 保证(x,y)合法，切grid[x][y]是没有被访问过的陆地
    void dfs(vector<vector<char>>& grid, int x, int y) {
        visited[x][y] = true;
        for (int i = 0; i < 4; ++i) {
            int newx = x + d[i][0];
            int newy = y + d[i][1];
            if (inArea(newx, newy) && !visited[newx][newy] && grid[newx][newy] == '1')
                dfs(grid, newx, newy);
        }
        return;
    }

public:
    int numIslands(vector<vector<char>> &grid) {
        m = grid.size();
        if (m == 0)
            return 0;

        n = grid[0].size();

        visited = vector<vector<bool>>(m, vector<bool>(n, false));

        int res = 0;
        for (int i = 0; i < m; ++i) {
            for (int j = 0; j < n; ++j) {
                if (grid[i][j] == '1' && !visited[i][j]) {
                    res++;
                    dfs(grid, i, j);
                }
            }
        }
        return res;
    }
};