HackerRank - My solution for https://www.hackerrank.com/contests/worldcodesprint/challenges/two-pluses

two-pluses.rb

## Solution for https://www.hackerrank.com/contests/worldcodesprint/challenges/two-pluses

# Read input from STDIN
n, m = gets.strip.split(' ').map{|x| x.to_i}
matrix = []
n.times do |row|
	matrix << gets.strip.split('')
end

# Calculate best area
def find_best_area(matrix)
	best_area = 1
	all_pluses = find_all_pluses(matrix).sort.reverse
	all_pluses.each do |plus|
		return best_area if (plus.area ** 2) <= best_area
		second_plus = find_second_plus(matrix, plus)
		best_area = [best_area, (plus.area * second_plus.area)].max
	end
	best_area
end

# Given a matrix, find all valid pluses
def find_all_pluses(matrix)
	all_pluses = []
	m, n = matrix.size, matrix[0].size
	max_plus_size = [n, m].min
	for j in 0..(m-1) do
		for i in 0..(n-1) do
			for plus_size in (1..max_plus_size).step(2)
				new_plus = Plus.new(j, i, plus_size)
				if new_plus.valid_on_matrix?(matrix)
					all_pluses << new_plus
				end
			end
		end
	end
	all_pluses
end

# Given a matrix and a plus, find a second valid plus
def find_second_plus(matrix, plus)
	blacken!(matrix, plus)
	second_plus = find_all_pluses(matrix).sort.last
	whiten!(matrix,plus)
	second_plus
end

# Change color of used cells to Bad
def blacken!(matrix, plus)
	colorize(matrix, plus, 'B')
end

# Revert Blacken on matrix
def whiten!(matrix, plus)
	colorize(matrix, plus, 'G')
end

# Change a color of particular element in matrix
def colorize(matrix, plus, color)
	plus.coords.each do |j, i|
		matrix[j][i] = color
	end
end

# Define object class for Plus
class Plus
	attr_reader :y, :x, :n
	def initialize(y, x, n)
		@x, @y, @n = x, y, n
	end

	def area
		(2 * n) - 1
	end

	def <=> (plus)
		n <=> plus.n
	end

	def half_length
		(n / 2).floor
	end

	def coords
		coords = ((y - half_length)..(y + half_length)).map{|j| [j, x]}
		coords += ((x - half_length)..(x + half_length)).map{|i| [y, i]}
	end
	
	def valid_on_matrix?(matrix)
		coords.each do |j, i|
			return false if (j < 0) || (i < 0) || matrix[j].nil? || matrix[j][i].nil?
			return false if matrix[j][i] == 'B'
		end	
		true
	end
end

puts find_best_area(matrix)