1require 'bundler/inline'
 2
 3gemfile do
 4  source 'https://rubygems.org'
 5
 6  gem 'minitest'
 7end
 8
 9require 'minitest/autorun'
10
11=begin
12A Dyck word is a sequence of +1’s and -1’s with the property that the sum of any prefix
13of the sequence is never negative.
14For example, +1,−1,+1,−1 is a Dyck word, but +1,−1,−1,+1 is not a Dyck word since the prefix +1 − 1 − 1 < 0.
15
16Describe any relationship between Dyck words and Stack push(x) and pop() operations.
17=end
18
19class Example < Minitest::Test
20  def dyck_word?(stack)
21    sum = 0
22    stack.each do |item|
23      return if sum.negative?
24      sum += item
25    end
26    true
27  end
28
29  def test_valid_word
30    assert dyck_word?([1, -1, 1, -1])
31  end
32
33  def test_invalid_word
34    refute dyck_word?([1, -1, -1, 1])
35  end
36end