# 200. Number of Islands

## problem description

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
```

## algorithm thought

这题不是很难，直接遍历所有，对每个为1的点，进行DFS遍历。没遍历到一个1的节点就赋值为0. 每次进行DFS的时候，将res+1.

## code

```cpp
class Solution {
public:
    int numIslands(vector<vector<char>>& grid) {
        int res=0;
        for(int i=0;i<grid.size();++i){
            for(int j=0;j<grid[0].size();++j){
                if(grid[i][j]=='1'){
                    DFS(grid,i,j);
                    res++;
                }
            }
        }
        return res;
    }
    void DFS(vector<vector<char>> &grid, int x, int y)
    {
        grid[x][y] = '0';
        if(x > 0 && grid[x - 1][y] == '1')
            DFS(grid, x - 1, y);
        if(x < grid.size() - 1 && grid[x + 1][y] == '1')
            DFS(grid, x + 1, y);
        if(y > 0 && grid[x][y - 1] == '1')
            DFS(grid, x, y - 1);
        if(y < grid[0].size() - 1 && grid[x][y + 1] == '1')
            DFS(grid, x, y + 1);
    }
};
```

## algorithm analysis

对于二维数组，每个节点最多访问两次，所以总的最多访问的次数是2\*n².最后时间复杂度O(n²)


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://kongchengzhuge.gitbook.io/leetcode/200.-number-of-islands.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.