Ruby practices

cavity_finder.rb

#!/bin/ruby

class CavityFinder
    def initialize(grid)
        @grid = grid
    end

    def cavities
        cavities = []
        i = 1
        while i < size - 1
            j = 1
            while j < size - 1
                cavities << [i, j] if is_cavity?(i, j)
                j += 1
            end
            i += 1
        end
        cavities
    end

    def replace_cavities_with(symbol)
        self.cavities.each do |coords|
            @grid[coords[0]][coords[1]] = symbol
        end
    end

    def is_cavity?(row, column)
        ary = []
        ary << @grid[row - 1][column]
        ary << @grid[row][column - 1]
        ary << @grid[row][column + 1]
        ary << @grid[row + 1][column]
        ary.max < @grid[row][column]
    end

    def size
        @size ||= @grid.size
    end

    def to_s
        @grid.each { |ary| puts ary.join }
    end
end

n = gets.strip.to_i
grid = Array.new(n)
for grid_i in (0..n-1)
    grid[grid_i] = gets.strip.split('').map(&:to_i)
end

cavity_finder = CavityFinder.new(grid)
cavity_finder.replace_cavities_with('X')
cavity_finder.to_s

stone_finder.rb

class StoneFinder
    attr_accessor :stones, :diff_a, :diff_b
    
    def initialize(stones, diff_a, diff_b)
        @stones = stones
        @diff_a = diff_a
        @diff_b = diff_b
    end
    
    def possible_last_values
        last_values = []
        last_values << (self.stones - 1) * self.diff_a
        (self.stones - 1).times do |n|
            new_val = last_values[0] - (n + 1) * self.diff_a + (n + 1) * self.diff_b
            last_values << new_val
        end
        last_values.uniq.sort
    end
end

num_tests = gets.strip.to_i
tests = []

i = 0
while i < num_tests
    num_stones = gets.strip.to_i
    diff_a = gets.strip.to_i
    diff_b = gets.strip.to_i
    tests << StoneFinder.new(num_stones, diff_a, diff_b)
    i += 1
end

tests.each do |test|
    puts test.possible_last_values.join(' ')
end