Number of Islands
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.
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","0","1","1","0"],
["1","0","1","0","0"],
["0","0","0","0","1"],
["0","0","0","1","1"]
]
Output: 3
Examples
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
Explanation: All connected '1' cells form a single island. There are no other disconnected land cells.
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
Explanation: 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).
Constraints
- -m == grid.length
- -n == grid[i].length
- -1 <= m, n <= 300
- -grid[i][j] is '0' or '1'.
Optimal Complexity
Time
O(m * n)
Space
O(m * n)
Practice this problem with an AI interviewer
TechInView conducts a full voice mock interview — the AI asks clarifying questions, evaluates your approach, watches you code, and scores you on 5 dimensions. Just like a real FAANG interview.
Start a free interview