矩阵中岛数量问题

package com.algorithm.practice.matrix;

public class IsLand {

    public static int countIslands(int[][] m) {
        if (m == null || m[0] == null) {
            return 0;
        }
        int N = m.length;//行数
        int M = m[0].length;//列数
        int res = 0;
        for (int i = 0; i < N; i++) {
            for (int j = 0; j < M; j++) {
                if (m[i][j] == 1) { //遍历找出每个是1的位置
                    res++;
                    infect(m, i, j, N, M);
                }
            }
        }
        return res;
    }

    public static void infect(int[][] m, int i, int j, int N, int M) { //将m矩阵(i,j)位置周边的1都改为2
        if (i < 0 || i >= N || j < 0 || j >= M || m[i][j] != 1) {
            return;//如果(i,j)不在矩阵中,退出
        }
        m[i][j] = 2;
        //将该点的上下左右为1的数继续改为2并递归
        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);
    }

    public static void main(String[] args) {
        int[][] m1 = {
                { 0, 0, 0, 0, 0, 0, 0, 0, 0 },
                { 0, 1, 1, 1, 0, 1, 1, 1, 0 },
                { 0, 1, 1, 1, 0, 0, 0, 1, 0 },
                { 0, 1, 1, 0, 0, 0, 0, 0, 0 },
                { 0, 0, 0, 0, 0, 1, 1, 0, 0 },
                { 0, 0, 0, 0, 1, 1, 1, 0, 0 },
                { 0, 0, 0, 0, 0, 0, 0, 0, 0 }, };
        System.out.println(countIslands(m1));

        int[][] m2 = {
                { 0, 0, 0, 0, 0, 0, 0, 0, 0 },
                { 0, 1, 1, 1, 1, 1, 1, 1, 0 },
                { 0, 1, 1, 1, 0, 0, 0, 1, 0 },
                { 0, 1, 1, 0, 0, 0, 1, 1, 0 },
                { 0, 0, 0, 0, 0, 1, 1, 0, 0 },
                { 0, 0, 0, 0, 1, 1, 1, 0, 0 },
                { 0, 0, 0, 0, 0, 0, 0, 0, 0 }, };
        System.out.println(countIslands(m2));

    }

}