# Number of Islands Solution (Leetcode 200) - Top Google Coding Interview Question Mani
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Number of Islands problem is one of the must learn interview questions to learn

Given an `m x n` 2d `grid` map of `'1'`s (land) and `'0'`s (water), return 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: grid = [
["1","1","1","1","0"],
["1","1","0","1","0"],
["1","1","0","0","0"],
["0","0","0","0","0"]
]
Output: 1
```

Example 2:

```Input: grid = [
["1","1","0","0","0"],
["1","1","0","0","0"],
["0","0","1","0","0"],
["0","0","0","1","1"]
]
Output: 3
```

Constraints:

• `m == grid.length`
• `n == grid[i].length`
• `1 <= m, n <= 300`
• `grid[i][j]` is `'0'` or `'1'`.

Solution:

``````1
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/**
* Time Complexity: O(mn)
* Space Complexity: O(mn)
*/

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

private void dfs(char[][] grid , int i, int j) {
if(i < 0 || i >= grid.length || j < 0 || j >= grid.length || grid[i][j] == '0') {
return;
}
grid[i][j] = '0';
dfs(grid, i+1, j);
dfs(grid, i-1, j);
dfs(grid, i, j+1);
dfs(grid, i, j-1);
}
}

``````
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