diff --git a/lib/problems.rb b/lib/problems.rb
index 5085953d..1a0d5677 100644
--- a/lib/problems.rb
+++ b/lib/problems.rb
@@ -1,9 +1,33 @@
 require_relative './stack.rb'
 
-# Time Complexity: ?
-# Space Complexity: ?
+# Time Complexity: O(n), where n is the length of the string
+# Space Complexity: O(n)
 def balanced(string)
-  raise NotImplementedError, "Not implemented yet"
+  # base case: length of the expression must be even
+    return false if string.length.odd?
+  
+  # create a new stack
+    stack = Stack.new
+  
+  # loop through each char in the string
+    string.each_char do |char|
+      # if current char is an opening brace, push the corresponding closing brace to the stack.
+      if char == "(" 
+        stack.push(")")
+      elsif char == "{"
+        stack.push("}")
+      elsif char == "["
+        stack.push("]")
+      # if current char is the same as the last inserted char on stack, remove last inserted char
+      elsif !stack.empty? && stack.top == char
+        stack.pop
+      else # if stack is empty or last inserted char is not an opening brace
+        return false
+      end
+    end
+  
+    # string is balanced only if stack is empty at this point
+    return stack.empty?
 end
 
 # Time Complexity: ?
diff --git a/lib/queue.rb b/lib/queue.rb
index 828217c6..97090c60 100644
--- a/lib/queue.rb
+++ b/lib/queue.rb
@@ -1,31 +1,64 @@
 class Queue
 
   def initialize
-    # @store = ...
-    raise NotImplementedError, "Not yet implemented"
+    @store = Array.new(20)
+    @size = 20
+    @front = -1
+    @rear = -1
   end
 
   def enqueue(element)
-    raise NotImplementedError, "Not yet implemented"
+    if (@front == 0 && @rear == @size - 1) ||
+      (@rear == (@front - 1) % (@size - 1))
+      raise RuntimeError.new("Error: Queue is full")
+    elsif @front == -1 # queue is empty
+      @front = @rear = 0
+      @store[@rear] = element
+    elsif @rear == @size - 1 && @front != 0 # rear needs to wrap around
+      @rear = 0
+      @store[@rear] = element
+    else 
+      @rear += 1
+      @store[@rear] = element
+    end
   end
 
   def dequeue
-    raise NotImplementedError, "Not yet implemented"
-  end
-
-  def front
-    raise NotImplementedError, "Not yet implemented"
-  end
-
-  def size
-    raise NotImplementedError, "Not yet implemented"
+    if @front == -1
+      raise RuntimeError.new("Error: The queue is empty")
+    end
+  
+    data = @store[@front]
+    @store[@front] = nil # overwrite the element being deleted
+  
+    if @front == @rear # queue is now empty
+      @front = @rear = -1
+    elsif @front == @size - 1 # front needs to wrap around
+      @front = 0
+    else
+      @front += 1
+    end
+  
+    return data
   end
 
   def empty?
-    raise NotImplementedError, "Not yet implemented"
+    return @front == @rear
   end
 
   def to_s
-    return @store.to_s
+    list = Array.new
+
+    @store[@front...@size].each do |element|
+      list.push(element) if element
+    end
+
+    if @rear < @front # the queue has wrapped around
+      @store[0..@rear].each do |element|
+        list.push(element) if element
+      end
+    end
+      
+    return list.to_s
   end
 end
diff --git a/lib/stack.rb b/lib/stack.rb
index cfc6ef0f..41d89fe7 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"
+    @store.remove_first
   end
 
   def empty?
-    raise NotImplementedError, "Not yet implemented"
+    @store.empty?
+  end
+
+  def top
+    @store.get_first
   end
 
   def to_s
diff --git a/test/problems_test.rb b/test/problems_test.rb
index 046f059e..694262b8 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
       
@@ -33,7 +33,7 @@
     end
   end
 
-  describe "postfix" do
+  xdescribe "postfix" do
     it "can add a 2 numbers together" do
       
       expect(evaluate_postfix("34+")).must_equal 7
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