diff --git a/lib/problems.rb b/lib/problems.rb
index 5085953d..9bf62535 100644
--- a/lib/problems.rb
+++ b/lib/problems.rb
@@ -1,13 +1,74 @@
+# frozen_string_literal: true
+
 require_relative './stack.rb'
 
-# Time Complexity: ?
-# Space Complexity: ?
+# Time Complexity: O(n)
+# # Space Complexity: O(n)
+# def balanced(string)
+#   opening_brackets = Stack.new
+#   # string.each_char do |char|
+#   #   # if an opening bracket, push it into the opening_brackets
+#   #   if char == '{' || char == '(' || char == '['
+#   #     opening_brackets.push(char)
+#   #   else
+
+#       # compare if the last opening bracket matches with the current closing bracket in char
+#       opening_bracket = opening_brackets.pop()
+#         if matching_braces == '}' && opening_bracket != '{'
+#           return false
+#         elsif matching_braces == ')' && opening_bracket != '('
+#           return false
+#         elsif matching_braces == ']' && opening_bracket != '['
+#           return false
+#         end
+
+#   # if everything matches/closes, then we have a balanced brackets
+#   return opening_brackets.empty?
+# end
+
 def balanced(string)
-  raise NotImplementedError, "Not implemented yet"
+  matching_hash = {
+    '{' => '}',
+    '(' => ')',
+    '[' => ']'
+  }
+  matching_stacks = Stack.new
+
+  string.each_char do |char|
+    if matching_hash.key? char
+      matching_stacks.push(char)
+    elsif matching_hash.value? char
+      closing_brace = matching_hash[matching_stacks.pop]
+      return false if closing_brace != char
+    end
+  end
+  matching_stacks.empty?
 end
 
-# Time Complexity: ?
-# Space Complexity: ?
+# Time Complexity: O(n)
+# Space Complexity: O(n)
 def evaluate_postfix(postfix_expression)
-  raise NotImplementedError, "Not implemented yet"
+  operands = Stack.new
+  postfix_expression.each_char do |char|
+    if char != '+' && char != '*' && char != '-' && char != '/'
+      operands.push(char)
+    else # if not pop 2 values
+      y = operands.pop.to_i
+      x = operands.pop.to_i
+      result = 0
+
+      if char == '+'
+        result = x + y
+      elsif char == '*'
+        result = x * y
+      elsif char == '-'
+        result = x - y
+      elsif char == '/'
+        result = x / y
+      end
+
+      operands.push(result)
+    end
+  end
+  operands.pop
 end
diff --git a/lib/queue.rb b/lib/queue.rb
index 828217c6..b7643c4c 100644
--- a/lib/queue.rb
+++ b/lib/queue.rb
@@ -1,31 +1,45 @@
-class Queue
+# frozen_string_literal: true
 
-  def initialize
-    # @store = ...
-    raise NotImplementedError, "Not yet implemented"
+class Queue
+  def initialize(size = 30)
+    @store = []
+    @size = size
+    @front = 0
+    @back = 0
   end
 
   def enqueue(element)
-    raise NotImplementedError, "Not yet implemented"
-  end
-
-  def dequeue
-    raise NotImplementedError, "Not yet implemented"
+    if @front == (@back + 1) % @size
+      raise StandardError, 'Queue is full.'
+    else
+      @store[@back] = element
+      @back = (@back + 1) % @size
+    end
   end
 
-  def front
-    raise NotImplementedError, "Not yet implemented"
+  def empty?
+    @front == @back # front & back are pointing to the same position, then we've read everything
   end
 
-  def size
-    raise NotImplementedError, "Not yet implemented"
-  end
+  def dequeue
+    return nil if empty?
 
-  def empty?
-    raise NotImplementedError, "Not yet implemented"
+    element = @store[@front]
+    @front = (@front + 1) % @size # advance the front pointer/index by 1
+    element
   end
 
   def to_s
-    return @store.to_s
+    return [] if empty?
+
+    queue = []
+    current = @front
+
+    while current != @back
+      queue << @store[current]
+      current = (current + 1) % @size
+    end
+
+    queue.to_s
   end
 end
diff --git a/lib/stack.rb b/lib/stack.rb
index cfc6ef0f..48c432c4 100644
--- a/lib/stack.rb
+++ b/lib/stack.rb
@@ -1,19 +1,22 @@
 class Stack
   def initialize
-    # @store = ...
-    raise NotImplementedError, "Not yet implemented"
+    @store = LinkedList.new
   end
 
   def push(element)
-    raise NotImplementedError, "Not yet implemented"
+    @store.add_first(element)
   end
 
-  def pop
-    raise NotImplementedError, "Not yet implemented"
+  def pop()
+    return nil if self.empty? 
+
+    element = @store.remove_first
+
+    return element
   end
 
-  def empty?
-    raise NotImplementedError, "Not yet implemented"
+  def empty?()
+    return @store.empty?
   end
 
   def to_s
diff --git a/test/problems_test.rb b/test/problems_test.rb
index 046f059e..7467bcad 100644
--- a/test/problems_test.rb
+++ b/test/problems_test.rb
@@ -2,7 +2,7 @@
 
 Minitest::Reporters.use! Minitest::Reporters::SpecReporter.new
 
-xdescribe "Test wave 3 problems" do
+describe "Test wave 3 problems" do
   describe "balanced" do
     it "Given balanced strings it should return true" do
       
@@ -36,20 +36,20 @@
   describe "postfix" do
     it "can add a 2 numbers together" do
       
-      expect(evaluate_postfix("34+")).must_equal 7
-      expect(evaluate_postfix("34*")).must_equal 12
-      expect(evaluate_postfix("34-")).must_equal(-1)
-      expect(evaluate_postfix("34/")).must_equal 0
+      expect(evaluate_postfix('34+')).must_equal 7
+      expect(evaluate_postfix('34*')).must_equal 12
+      expect(evaluate_postfix('34-')).must_equal(-1)
+      expect(evaluate_postfix('34/')).must_equal 0
     end
 
     it "can add a evaluate a more complicated expression" do
       
-      expect(evaluate_postfix("34+2*")).must_equal 14
-      expect(evaluate_postfix("34*2/")).must_equal 6
-      expect(evaluate_postfix("34-1+")).must_equal 0
-      expect(evaluate_postfix("34/7-")).must_equal(-7)
-      expect(evaluate_postfix("35+6*")).must_equal 48
-      expect(evaluate_postfix("62/5+")).must_equal 8
+      expect(evaluate_postfix('34+2*')).must_equal 14
+      expect(evaluate_postfix('34*2/')).must_equal 6
+      expect(evaluate_postfix('34-1+')).must_equal 0
+      expect(evaluate_postfix('34/7-')).must_equal(-7)
+      expect(evaluate_postfix('35+6*')).must_equal 48
+      expect(evaluate_postfix('62/5+')).must_equal 8
     end
   end
 end
\ No newline at end of file
diff --git a/test/stack_test.rb b/test/stack_test.rb
index cb15a12c..7c0fa755 100644
--- a/test/stack_test.rb
+++ b/test/stack_test.rb
@@ -9,6 +9,7 @@
   it "pushes something onto a empty Stack" do
     s = Stack.new
     s.push(10)
+    s.empty?.must_equal false
     expect(s.pop).must_equal 10
   end
 
diff --git a/test/test_helper.rb b/test/test_helper.rb
index 426168c8..f9340080 100644
--- a/test/test_helper.rb
+++ b/test/test_helper.rb
@@ -10,4 +10,4 @@
 require_relative "../lib/queue.rb"
 require_relative "../lib/stack.rb"
 # Extra exercises
-# require_relative "../lib/problems.rb"
\ No newline at end of file
+require_relative "../lib/problems.rb"
\ No newline at end of file