MediumGraphsFree practice
Number of Islands
Asked atamazongooglemicrosoftbloomberglinkedin
01 · Problem
Given an m x n 2D binary grid grid which represents a 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.
02 · Examples
Example 01
Input
grid = [["1","1","1","1","0"],["1","1","0","1","0"],["1","1","0","0","0"],["0","0","0","0","0"]]
Output
1
All connected '1' cells form a single island. There are no other disconnected land cells.
Example 02
Input
grid = [["1","1","0","0","0"],["1","1","0","0","0"],["0","0","1","0","0"],["0","0","0","1","1"]]
Output
3
There are three separate groups of connected land cells: top-left 2x2 block, center cell at (2,2), and bottom-right two cells at (3,3) and (3,4).
03 · Constraints
- 01m == grid.length
- 02n == grid[i].length
- 031 <= m, n <= 300
- 04grid[i][j] is '0' or '1'.
04 · Optimal complexity
- Time
- O(m * n)
- Space
- O(m * n)
05 · Two ways to work on it
Practice it alone or rehearse it as an interview.
Practice Mode gives you an editor and test runs, nothing else. AI Interview Mode puts a voice interviewer on the other side, adds a clock, and ends with a scored summary of the round.