diff --git a/lib/problems.rb b/lib/problems.rb
index 5085953d..732c6de0 100644
--- a/lib/problems.rb
+++ b/lib/problems.rb
@@ -1,13 +1,39 @@
 require_relative './stack.rb'
 
-# Time Complexity: ?
-# Space Complexity: ?
+# Time Complexity: O(n)
+# Space Complexity: O(n)
 def balanced(string)
-  raise NotImplementedError, "Not implemented yet"
+  stack = Stack.new
+  string.each_char do |char|
+    if char == "(" || char == "[" || char == "{"
+      stack.push(char)
+    elsif char == ")" || char == "]" || char == "}"
+      last_pushed = stack.pop()
+      if last_pushed == "(" && char != ")" || 
+        last_pushed == "[" && char != "]" ||
+        last_pushed == "{" && char != "}"
+        return false
+      end
+    end 
+  end
+
+  # check if stack is full of opening braces
+  return stack.empty?
 end
 
-# Time Complexity: ?
-# Space Complexity: ?
+# Time Complexity: O(n)
+# Space Complexity: O(n)
 def evaluate_postfix(postfix_expression)
-  raise NotImplementedError, "Not implemented yet"
+  stack = Stack.new
+  postfix_expression.each_char do |char|
+    if /[0-9]/.match(char)
+      stack.push(char.to_i)
+    elsif /[\+\-\/\*]/.match(char)
+      second_operand = stack.pop()
+      first_operand = stack.pop()
+      stack.push(first_operand.send(char, second_operand))
+    end
+  end
+
+  return stack.pop()
 end
diff --git a/lib/queue.rb b/lib/queue.rb
index 828217c6..3c461685 100644
--- a/lib/queue.rb
+++ b/lib/queue.rb
@@ -1,31 +1,57 @@
 class Queue
 
+  ARRAY_SIZE = 20
+
   def initialize
-    # @store = ...
-    raise NotImplementedError, "Not yet implemented"
+    @store = Array.new(ARRAY_SIZE)
+    @front = @back = 0
   end
 
   def enqueue(element)
-    raise NotImplementedError, "Not yet implemented"
+    if self.full?
+      raise NoMemoryError, "Array is full"
+    else
+      @store[@back] = element
+      @back = (@back + 1) % ARRAY_SIZE
+    end
   end
 
   def dequeue
-    raise NotImplementedError, "Not yet implemented"
+    if self.empty?
+      raise IndexError, "Array is empty"
+    else
+      dequeued_element = @store[@front]
+      @store[@front] = nil
+      @front = (@front + 1) % ARRAY_SIZE
+      return dequeued_element
+    end
   end
 
   def front
-    raise NotImplementedError, "Not yet implemented"
+    return @store[@front]
   end
 
   def size
-    raise NotImplementedError, "Not yet implemented"
+    return @front < @back ? @back - @front : @store.length + @back - @front
   end
 
   def empty?
-    raise NotImplementedError, "Not yet implemented"
+    return @front == @back
+  end
+
+  def full?
+    return @front == (@back + 1) % ARRAY_SIZE
   end
 
   def to_s
-    return @store.to_s
+    string = "["
+    i = @front
+    until i == @back - 1
+      string << "#{@store[i]}, "
+      i = (i + 1) % ARRAY_SIZE
+    end
+
+    string << "#{@store[@back - 1]}]"
+    return string
   end
 end
diff --git a/lib/stack.rb b/lib/stack.rb
index cfc6ef0f..c262e900 100644
--- a/lib/stack.rb
+++ b/lib/stack.rb
@@ -1,19 +1,18 @@
 class Stack
   def initialize
-    # @store = ...
-    raise NotImplementedError, "Not yet implemented"
+    @store = LinkedList.new
   end
 
   def push(element)
-    raise NotImplementedError, "Not yet implemented"
+    @store.add_last(element)
   end
 
   def pop
-    raise NotImplementedError, "Not yet implemented"
+    @store.remove_last()
   end
 
   def empty?
-    raise NotImplementedError, "Not yet implemented"
+    return @store.empty?
   end
 
   def to_s
diff --git a/test/problems_test.rb b/test/problems_test.rb
index 046f059e..f28aff57 100644
--- a/test/problems_test.rb
+++ b/test/problems_test.rb
@@ -1,8 +1,9 @@
 require_relative 'test_helper'
+require_relative '../lib/problems'
 
 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