#0267

Number of Islands

MediumDSA28 min20 XP

Problem

Given a grid of "1" land and "0" water, return the number of islands. Land connects horizontally and vertically, not diagonally.

Why This Matters

Grid DFS/BFS is the bridge between matrix problems and graph traversal.

Function Signature

def num_islands(grid):

Examples

Example 1

Inputgrid = [["1","1","0"],["0","1","0"],["1","0","1"]]

Output3

There is one connected island at top-left, plus two isolated land cells.

Constraints

Return the exact requested value.
Handle edge cases cleanly.
Use the intended DSA pattern when brute force would scale poorly.

CodePython

def num_islands(grid):
    """
    Input example: grid = [["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 num_islands(grid):
    """
    Input example: grid = [["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.

Testcases2 visible / 3 hidden categories

Three islands

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

Expected3

Diagonal land does not connect.

All water

Input[["0","0"],["0","0"]]

Expected0

There is no land to start a traversal.

Hidden Test Categories

all landsingle rowempty grid