200_number_of_islands.rb

# Leetcode Problem: https://leetcode.com/problems/number-of-islands/
#Given a 2d grid map of '1's (land) and '0's (water), count 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
# ----------------------------------------------------------------------------------------------------------------------
# @param {Character[][]} grid
# @return {Integer}
def num_islands(grid)
  return 0 if grid.nil?

  number_of_rows = grid.length
  return 0 if number_of_rows.zero?

  number_of_columns = grid[0].length
  return 0 if number_of_columns.zero?

  number_of_islands = 0
  number_of_rows.times.each do |row_index|
    number_of_columns.times.each do |column_index|
      element = grid[row_index][column_index]
      next if element == '0'

      if element == '1'
        number_of_islands += 1
        grid = mark_adjacents_as_visited(grid, row_index, column_index)
      end
    end
  end

  number_of_islands
end

def mark_adjacents_as_visited(grid, row_index, column_index)
  number_of_rows = grid.length
  number_of_columns = grid[0].length

  if row_index.negative? || row_index >= number_of_rows || column_index.negative? || column_index >= number_of_columns
    return grid
  end

  element = grid[row_index][column_index]
  return grid if element == '0'

  grid[row_index][column_index] = '0'
  grid = mark_adjacents_as_visited(grid, row_index - 1, column_index) #top
  grid = mark_adjacents_as_visited(grid, row_index + 1, column_index) #bottom
  grid = mark_adjacents_as_visited(grid, row_index, column_index - 1) #Left
  grid = mark_adjacents_as_visited(grid, row_index, column_index + 1) #right

  grid
end

p num_islands(
      [['1','1','1','1','0'],['1','1','0','1','0'],['1','1','0','0','0'],['0','0','0','0','0']]
  )