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.lengthn == grid[i].length1 <= m, n <= 300grid[i][j]is'0'or'1'.
Solution:
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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[0].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[0].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);
}
}