200_Number_of_Islands

Number of Islands

Difficulty Medium

tags DFS

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

思路：

这道题考虑了下union find, 但是能用DFS实现~N的解法就不要去自寻烦恼

要注意的Case:

1. DFS写的时候能用递归就用递归好了和用队列在空间上是一样的， 而用队列你还需要多写很多行。

solution 1

class Solution {
    void dfsmarker(vector<vector<char>>& grid, int i, int j) {
        if (i < 0 || j < 0 || i >= grid.size() || j >= grid[0].size() || grid[i][j] == '0') return;
        grid[i][j] = '0';
        dfsmarker(grid, i+1, j);
        dfsmarker(grid, i-1, j);
        dfsmarker(grid, i, j+1);
        dfsmarker(grid, i, j-1);
        return;
    }
public:
    int numIslands(vector<vector<char>>& grid) {
        int res=0;
        if (grid.size() == 0) return res;
        vector<vector<int>> num(grid.size(), vector<int>(grid[0].size(), false));
        for (int i = 0; i<grid.size(); i++)
            for (int j = 0; j<grid[0].size(); j++)
                if (grid[i][j] == '1'){
                    res++;
                    dfsmarker(grid, i, j);    
                }
        return res;
    }
};