diff --git a/lib/problems.rb b/lib/problems.rb
index 5085953d..87809b8c 100644
--- a/lib/problems.rb
+++ b/lib/problems.rb
@@ -1,13 +1,63 @@
 require_relative './stack.rb'
 
-# Time Complexity: ?
-# Space Complexity: ?
+# Time Complexity: O(n) - where n is the number of characters in the string
+# Space Complexity: O(n) - worse case, all characters are openers and we push them all into the stack. 
 def balanced(string)
-  raise NotImplementedError, "Not implemented yet"
+  brackets = {
+    "(" => ")",
+    "{" => "}",
+    "[" => "]"
+  }
+
+  openers = brackets.keys
+  closers = brackets.values
+
+  opener_stack = []
+
+  string.each_char do |char|
+    if openers.include?(char)
+      opener_stack.push(char)
+    end
+
+    if closers.include?(char)
+      if opener_stack.empty?
+        return false
+      else
+        removed_opener = opener_stack.pop
+        if char != brackets[removed_opener]
+          return false
+        end
+      end
+    end
+  end
+
+  return opener_stack.empty?
+
 end
 
-# Time Complexity: ?
-# Space Complexity: ?
+# Time Complexity: O(n) - where n is the number of characters in the expression 
+# Space Complexity: O(n) - We're using a stack to store values. 
 def evaluate_postfix(postfix_expression)
-  raise NotImplementedError, "Not implemented yet"
+  stack = []
+  postfix_expression.each_char do |element|
+    if element == element.to_i.to_s
+      stack.push(element.to_i)
+    else
+      b = stack.pop
+      a = stack.pop 
+      case element
+      when "+"
+        stack.push(a + b)
+      when "-"
+        stack.push(a - b)
+      when "*"
+        stack.push(a * b)
+      when "/"
+        stack.push(a / b)
+      end
+    end
+  end
+  return stack.pop
 end
+
+
diff --git a/lib/queue.rb b/lib/queue.rb
index 828217c6..20ea63fd 100644
--- a/lib/queue.rb
+++ b/lib/queue.rb
@@ -1,31 +1,60 @@
 class Queue
-
+  MAX_SIZE = 20
   def initialize
-    # @store = ...
-    raise NotImplementedError, "Not yet implemented"
+    @list = Array.new(MAX_SIZE)
+    @front = -1
+    @back = -1
   end
 
   def enqueue(element)
-    raise NotImplementedError, "Not yet implemented"
+    if (@back == MAX_SIZE - 1 && @front == 0) || (@back == (@front - 1) % (MAX_SIZE - 1))
+      raise "queue is full"
+    elsif @front == -1 #  queue is empty?
+      @front = 0
+      @back = 1 
+      @list[@front] = element
+    else
+      @list[@back] = element
+      @back = (@back + 1) % MAX_SIZE
+    end
+
   end
 
   def dequeue
-    raise NotImplementedError, "Not yet implemented"
+
+    if @front == -1
+      raise "queue is empty"
+    end
+
+    data = @list[@front]
+
+    if @front == @back  # if the queue is now empty
+      @front = -1
+      @back = -1
+    else
+      @front = (@front + 1) % MAX_SIZE
+    end
+
+    return data
   end
 
   def front
-    raise NotImplementedError, "Not yet implemented"
+    return @list[@front]
   end
 
   def size
-    raise NotImplementedError, "Not yet implemented"
+    return @front == -1 ? 0 : @back - @front + 1
   end
 
   def empty?
-    raise NotImplementedError, "Not yet implemented"
+    return @front == -1 || @front == @back
   end
 
   def to_s
-    return @store.to_s
+    if @front <= @back 
+      return @list.slice(@front...@back).to_s 
+    else
+      return (@list.slice(@front...MAX_SIZE) + @list.slice(0...@back)).to_s
+    end
   end
 end
diff --git a/lib/stack.rb b/lib/stack.rb
index cfc6ef0f..018504bd 100644
--- a/lib/stack.rb
+++ b/lib/stack.rb
@@ -1,19 +1,18 @@
 class Stack
   def initialize
-    # @store = ...
-    raise NotImplementedError, "Not yet implemented"
+    @list = LinkedList.new
   end
 
   def push(element)
-    raise NotImplementedError, "Not yet implemented"
+    @list.add_first(element)
   end
 
   def pop
-    raise NotImplementedError, "Not yet implemented"
+    @list.remove_first
   end
 
   def empty?
-    raise NotImplementedError, "Not yet implemented"
+    @list.empty?
   end
 
   def to_s
diff --git a/test/problems_test.rb b/test/problems_test.rb
index 046f059e..c0cfc618 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
       
diff --git a/test/test_helper.rb b/test/test_helper.rb
index 426168c8..6dfc6980 100644
--- a/test/test_helper.rb
+++ b/test/test_helper.rb
@@ -9,5 +9,4 @@
 require_relative "../lib/linked_list.rb"
 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