How should I practice Count Islands in a Binary Grid?

Start by reading the full statement, solve the visible examples, then compare the pattern against related Apple phone screen problems in FastPrep.

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FastPrep tags this as a Apple phone screen coding practice problem when the problem catalog has that company and stage context.

Problem · Array

Count Islands in a Binary Grid

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Problem statement

Given a rectangular binary grid, return the number of islands.

Each cell is either 1 for land or 0 for water. An island is a maximal group of land cells connected vertically or horizontally. Diagonal cells are not connected.

Function

countIslands(grid: int[][]) → int

Examples

Example 1

grid = [[1,1,0,0],[1,0,0,1],[0,0,1,1],[0,0,0,0]]return = 2

The land in the upper-left forms one island. The three connected land cells on the right form a second island.

Example 2

grid = [[0,0,0],[0,0,0]]return = 0

The grid contains no land, so it contains no islands.

Example 3

grid = [[1,0,1],[0,1,0],[1,0,1]]return = 5

Every land cell touches the others only diagonally. Because diagonal contact does not connect islands, all five land cells are separate.

Constraints

1 <= grid.length <= 300.
1 <= grid[r].length <= 300 for every row r.
All rows have the same length.
Every cell is either 0 or 1.
The caller does not require the input grid to remain unchanged.