Twitter Interview Question

README.md

This code visualizes the solution to this Twitter interview question.

My first solution only looked at immediate neighbors to figure out if you're in a trough that will collection water. The clever part is considering all values to the left and right of you, not immediate neighbors. My naive solution to [5,1,5] was pretty close, but as I expanded to more complex problems, considering your neighbors wasn't enough and my solution began to bloat. Looking at this answer gave me the insight to strip down my solution.

Once I had it solved I went back and built the simple solution display.

To run the tests simply run $> ruby -Itest rain_test.rb

rain.rb

# encoding: utf-8
class Rain
  attr_reader :land, :volume
  def initialize(land)
    @land = land
    @volume = 0
  end

  # from where you are you want to know if you're in a trough
  # Moving left find the largest value, moving right find the largest value
  # take the lesser of the two and subtract from yourself.
  def calculate_volume
    land.each_with_index{|h, index|
      left = land[0..index].max
      right = land[index..-1].max 
      if h < left && h < right
        @volume += [left,right].min - h
      end
    }
    return @volume
  end

  def print_solution
    puts "\nBoard for #{land.inspect}"
    max = land.max
    max.times do |n|
      n = max - n
      l = []
      land.each_with_index do |h, index|
        left = land[0..index].max
        right = land[index..-1].max 
        if h < left && h < right
          if n <= [left,right].min && n > h
            l << "•" 
          else
            l << "|" if n <= h
            l << " " if h < n
          end
        else
          l << "|" if n <= h
          l << " " if h < n
        end
      end
      puts l.join
    end
    puts "_" * land.length
    puts "Volume: #{calculate_volume}"
  end

end

rain_test.rb

require 'minitest/autorun'
require_relative 'rain'

describe Rain do
    specs =   { 
      [1,2,3,4,5,4,5,3] => 1,
      [2,1,2,3,4,2] => 1,
      [5,1,5] => 4,
      [2,7,2,1,7,4] => 11,
      [2,8,2,1,7,4] => 11,
      [2,7,2,-1,4,9] => 5+8+3,
      [5,1,0,1] => 1,
      [2,5,1,2,3,4,7,7,6,3,5] => 12
    }
    specs.each do |array, expected|
      it "should provide #{expected} for #{array}" do
        rain = Rain.new(array)
        rain.print_solution
        assert_equal expected, rain.volume, "Failed for #{array.inspect}"
      end
    end
end