TheAlgorithms-Ruby/searches/number_of_islands.rb

#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'.




#DFS, Recursive Bottom Up Approach - O(n*m) Time / O(1) Space
#Init num_of_islands = 0, return if the grid is empty
#Start a double loop with index to iterate through each plot (each value is a plot of either water or land in this case)
#if the plot is land, dfs(grid, x, y)
#num_of_islands += 1
#Return num_of_islands

#dfs(grid, x, y)
#Return if x or y are out of bounds, or if the plot is water
#Make the current plot water
#Call dfs again for up, down, left, and right     

# @param {Character[][]} grid
# @return {Integer}
def num_islands(grid)
    return 0 if grid.empty?
    
    #init num of islands
    islands = 0
    
    #loop through each element (plot) in the 2d array
    grid.each_with_index do |row, x|
      row.each_with_index do |plot, y|
        #if the plot is water, start a dfs
        if plot == "1"
          dfs(grid, x, y)
          #add 1 to islands once all connected land plots are searched
          islands += 1
        end
      end
    end
    
    #return ans
    islands
end

def dfs(grid, x, y)
    #don't search if out of bounds, or if it's already water
    return if x < 0 || x >= grid.length || y < 0 || y >= grid[0].length || grid[x][y] == "0"
    
    #set the plot to water
    grid[x][y] = "0"
    
    #search each adjacent plot
    dfs(grid, x - 1, y) #up
    dfs(grid, x + 1, y) #down
    dfs(grid, x, y - 1) #left
    dfs(grid, x, y + 1) #right
end