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:

Input:
11110
11010
11000
00000

Output: 1

Example 2:

Input:
11000
11000
00100
00011

Output: 3

解析

此題要算出在一個由0與1組成的二維矩陣中，有幾個由數字1組成的區塊。

解法

public class Solution {

    private int n;
    private int m;

    public int numIslands(char[][] grid) {
        int count = 0;
        n = grid.length;
        if (n == 0) return 0;
        m = grid[0].length;
        for (int i = 0; i < n; i++){
            for (int j = 0; j < m; j++)
                if (grid[i][j] == '1') {
                    DFSMarking(grid, i, j);
                    ++count;
                }
        }    
        return count;
    }

    private void DFSMarking(char[][] grid, int i, int j) {
        if (i < 0 || j < 0 || i >= n || j >= m || grid[i][j] != '1') return;
        grid[i][j] = '0';
        DFSMarking(grid, i + 1, j);
        DFSMarking(grid, i - 1, j);
        DFSMarking(grid, i, j + 1);
        DFSMarking(grid, i, j - 1);
    }
}

備註

希望透過記錄解題的過程，可以對於資料結構及演算法等有更深一層的想法。
如有需訂正的地方歡迎告知，若有更好的解法也歡迎留言，謝謝。