#0197

Count Islands in a Grid

MediumDSA25 min20 XP

Problem

Given a grid of 1s and 0s, count how many connected groups of 1s exist using 4-direction movement.

Why This Matters

Grid DFS is the bridge between simple graph traversal and real matrix problems like image regions, flood fill, and map analysis.

Function Signature

def count_islands(grid):

Examples

Example 1

Input[[1,1,0],[0,1,0],[1,0,1]]

Output3

The top-left land cells form one island, and the two isolated bottom cells form two more.

Constraints

Return the exact requested result.
Handle common edge cases.
Keep the solution readable before optimizing.

CodePython

def count_islands(grid):
    """
    Input example: [[1,1,0],[0,1,0],[1,0,1]]
    Return: 3
    Return the value from the function so the tests can compare it. Do not print unless the prompt asks.
    """
    # write your code here
    return 0

def count_islands(grid):
    """
    Input example: [[1,1,0],[0,1,0],[1,0,1]]
    Return: 3
    Return the value from the function so the tests can compare it. Do not print unless the prompt asks.
    """
    # write your code here
    return 0

Visible browser tests run here when available.

Testcases1 visible / 3 hidden categories

Core example

Input[[1,1,0],[0,1,0],[1,0,1]]

Expected3

The top-left land cells form one island, and the two isolated bottom cells form two more.

Hidden Test Categories

all waterone large islandthin diagonal-only land that should not connect