LeetCode-200-岛屿数量

200. 岛屿数量

难度中等1203收藏分享切换为英文接收动态反馈

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'

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image-20210622101310924

1所到之处infect 成2，增加一个岛

image-20210622101623372 image-20210622101658677 image-20210622101728451

class Solution {
   public static int numIslands(char[][] m) {
        if (m == null || m.length == 0 || m[0] == null || m[0].length == 0) {
            return 0;
        }
        int N = m.length;
        int M = m[0].length;
        int res = 0;
        for (int i = 0; i < N; i++) {
            for (int j = 0; j < M; j++) {
                if (m[i][j] == '1') {
                    res++;
                    infect(m, i, j, N, M);
                }
            }
        }
        return res;
    }

    // 目前来到m[i][j], 经历上下左右的感染过程
    public static void infect(char[][] m, int i, int j, int N, int M) {
        if (i < 0 || i >= N || j < 0 || j >= M || m[i][j] != '1') {
            return;
        }
        m[i][j] = '2';
        infect(m, i + 1, j, N, M);
        infect(m, i - 1, j, N, M);
        infect(m, i, j + 1, N, M);
        infect(m, i, j - 1, N, M);
    }
}

image-20210622101822191