LeetCode-in-All

200. Number of Islands

Medium

Given an m x n 2D binary grid grid which represents a map of '1's (land) and '0's (water), return the number of islands.

An island is surrounded by water and is formed by connecting adjacent lands horizontally or vertically. You may assume all four edges of the grid are all surrounded by water.

Example 1:

Input: grid = [

[“1”,”1”,”1”,”1”,”0”],

[“1”,”1”,”0”,”1”,”0”],

[“1”,”1”,”0”,”0”,”0”],

[“0”,”0”,”0”,”0”,”0”]

]

Output: 1

Example 2:

Input: grid = [

[“1”,”1”,”0”,”0”,”0”],

[“1”,”1”,”0”,”0”,”0”],

[“0”,”0”,”1”,”0”,”0”],

[“0”,”0”,”0”,”1”,”1”]

]

Output: 3

Constraints:

• m == grid.length
• n == grid[i].length
• 1 <= m, n <= 300
• grid[i][j] is '0' or '1'.

Solution

defmodule Solution do
  @spec num_islands(grid :: [[char]]) :: integer
  def num_islands(grid) do
    lands = grid |> Enum.with_index() |> Enum.reduce(MapSet.new(), fn {line, i}, lands ->
      line |> Enum.with_index() |> Enum.reduce(lands, fn {is_land?, j}, lands ->
        if is_land? == ?0 do
          lands
        else
          MapSet.put(lands, {i, j})
        end
      end)
    end)

    lands |> Enum.reduce({MapSet.new(), 0}, fn land, {visited, count}->
      if MapSet.member?(visited, land) do
        {visited, count}
      else
        visited = visit_island(visited, lands, land)
        {visited, count + 1}
      end
    end) |> elem(1)
  end

  defp visit_island(visited, lands, {i, j} = land) do
    with true <- MapSet.member?(lands, land),
         false <- MapSet.member?(visited, land) do
      visited
      |> MapSet.put(land)
      |> visit_island(lands, {i - 1, j})
      |> visit_island(lands, {i + 1, j})
      |> visit_island(lands, {i, j - 1})
      |> visit_island(lands, {i, j + 1})
    else
      _ -> visited
    end
  end
end

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