#0151

Count Islands in a Grid

MediumDSA25 min20 XP

Problem

Given a grid of 1s and 0s, count connected groups of 1s using four-direction movement.

Why This Matters

Grid DFS is used in image masks, maps, flood fill, and many interview graph problems.

Function Signature

def count_islands(grid):

Examples

Example 1

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

Output3

The top-left shape is one island; the two isolated 1s are separate islands.

Constraints

Return the exact requested value.
Handle empty or small inputs when the prompt allows them.
Keep the code readable and deterministic.

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 shape is one island; the two isolated 1s are separate islands.

Hidden Test Categories

empty or smallest inputduplicates or repeated valueslarger realistic input