#Leetcode# 200. Number of Islands

https://leetcode.com/problems/number-of-islands/

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

代码：

class Solution {
public:
    int numIslands(vector<vector<char>>& grid) {
        if(grid.empty() || grid[0].empty()) return 0;
        int n = grid.size(), m = grid[0].size();
        int ans = 0;
        vector<vector<bool> > flag(n, vector<bool> (m, false));
        for(int i = 0; i < n; i ++) {
            for(int j = 0; j < m; j ++) {
                if(grid[i][j] == '1' && !flag[i][j]) {
                    dfs(grid, flag, i, j);
                    ans ++;
                }
            }
        }
        return ans;
    }
    void dfs(vector<vector<char> > &grid, vector<vector<bool> > &flag, int x, int y) {
        if(x < 0 || x >= grid.size()) return;
        if(y < 0 || y >= grid[0].size()) return;
        if(grid[x][y] != '1' || flag[x][y]) return;
        flag[x][y] = true;
        dfs(grid, flag, x - 1, y);
        dfs(grid, flag, x + 1, y);
        dfs(grid, flag, x, y - 1);
        dfs(grid, flag, x, y + 1);
    }
};

　　求连续区域的个数 $flag$ 记录是不是被访问过 对于每一个值是 $1$ 的上下左右 $dfs$ 把和它连着的 $1$ 都标记住 搜索完 $ans$ 加一  $dfs$ 还是要好好学的哇