【LeetCode】Number of Islands 解题报告

【题目】

Given a 2d grid map of '1's (land) and '0's (water), count the number of islands. An island is surrounded by water and is formed by connecting adjacent lands horizontally or vertically. You may assume all four edges of the grid are all surrounded by water.

Example 1:

11110
11010
11000
00000

Answer: 1

Example 2:

11000
11000
00100
00011

Answer: 3

【解析】

题意：一个只包含字符0和1的二维数组，找出里面不相邻的只包含1的块的个数。

思路：DFS、BFS。只要遍历一遍，碰到一个1，就把它周围所有相连的1都标记为非1，这样整个遍历过程中碰到的1的个数就是所求解。

【Java代码：DFS、递归】

public class Solution {
    private int m, n;
    
    public int numIslands(char[][] grid) {
        m = grid.length;
        if (m == 0) return 0;
        n = grid[0].length;
        if (n == 0) return 0;
        
        int ans = 0;
        for (int i = 0; i < m; i++) {
            for (int j = 0; j < n; j++) {
                if (grid[i][j] != '1') continue;
                
                ans++;
                dfs(grid, i, j);
            }
        }
        return ans;
    }
    
    
    public void dfs(char[][] grid, int i, int j) {
        if (i < 0 || i >= m || j < 0 || j >= n) return;
        
        if (grid[i][j] == '1') {
            grid[i][j] = '2';
            dfs(grid, i - 1, j);
            dfs(grid, i + 1, j);
            dfs(grid, i, j - 1);
            dfs(grid, i, j + 1);
        }
    }
}