Commit f768b7a

@@ -0,0 +1,51 @@
+require 'bundler/inline'
+
+gemfile do
+  source 'https://rubygems.org'
+
+  gem 'minitest'
+end
+
+require 'minitest/autorun'
+
+=begin
+A matched string is a sequence of {, }, (, ), [, and ] characters that are properly matched.
+For example, “{{()[]}}” is a matched string, but this “{{()]}” is not, since the second { is matched with a ].
+Show how to use a stack so that, given a string of length n, you can determine if it is a matched string in O(n) time.
+=end
+
+class Example < Minitest::Test
+  def matches?(open, close)
+    case open
+    when '('
+      return close == ')'
+    when '{'
+      return close == '}'
+    when '['
+      return close == ']'
+    else
+      raise [open, close].inspect
+    end
+  end
+
+  def matched_string?(string)
+    stack = []
+    string.chars.each do |char|
+      case char
+      when '{', '[', '('
+        stack.push(char)
+      else
+        return unless matches?(stack.pop, char)
+      end
+    end
+    stack.size.zero?
+  end
+
+  def test_valid
+    assert matched_string?("{{()[]}}")
+  end
+
+  def test_invalid
+    refute matched_string?("{{()]}")
+  end
+end