Count Islands in Messenger Grid | Dataford Interview Questions

In a simplified Meta Messenger map, a 2D grid represents water and land, where '1' is land and '0' is water. Count how many distinct islands exist.

An island is a group of horizontally or vertically adjacent land cells. Cells connected diagonally do not belong to the same island.

Formal Specification

Write a function that takes grid, a list of lists of single-character strings ('0' or '1'), and returns an integer: the number of islands.

Constraints

1 <= len(grid) <= 300
1 <= len(grid[0]) <= 300
grid[i][j] is either '0' or '1'
Cells are connected only in 4 directions: up, down, left, right
You may modify the input grid in place

Function Signature

def num_islands(grid):

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