diff --git a/lib/problems.rb b/lib/problems.rb
index 5085953d..9b284664 100644
--- a/lib/problems.rb
+++ b/lib/problems.rb
@@ -1,13 +1,37 @@
 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 
+  twosomes = {
+    "[" => "]",
+    "(" => ")",
+    "{" => "}"
+  }
+
+  string.each_char do |char|
+    if twosomes.values.include?(char)
+      return false if twosomes.key(char) != stack.pop
+    elsif twosomes.key?(char)
+      stack.push(char)
+    end 
+  end
 end
 
-# Time Complexity: ?
-# Space Complexity: ?
+# Time Complexity: O(n)
+# Space Complexity: O(n)
 def evaluate_postfix(postfix_expression)
-  raise NotImplementedError, "Not implemented yet"
+  postfix_expression.each_char do |char|
+    if char.match(/\d/)
+      stack.push(char.to_i)
+    else 
+      front = stack.pop
+      back = stack.pop
+      evaluation = #{front} #{char} #{back}
+      stack.push(evaluation.to_i)
+    end 
+  end 
+
+  return stack.pop 
 end
diff --git a/lib/queue.rb b/lib/queue.rb
index 828217c6..4dd48bb1 100644
--- a/lib/queue.rb
+++ b/lib/queue.rb
@@ -1,31 +1,67 @@
 class Queue
 
   def initialize
-    # @store = ...
-    raise NotImplementedError, "Not yet implemented"
+    @queue = Array.new
+    @size = 20
+    @front = -1
+    @back = -1
   end
 
   def enqueue(element)
-    raise NotImplementedError, "Not yet implemented"
+    if @front == -1
+      @front = 0
+      @queue[@front] = element
+      @back = 1
+    elsif @front == @back
+      raise ArgumentError, "The queue is full; cannot enqueue at this time."
+    else
+      @queue[@back] = element
+      @back = (1 + @back) % @size 
+    end
   end
 
   def dequeue
-    raise NotImplementedError, "Not yet implemented"
+    if @front == -1
+      raise ArgumentError, "The queue is empty."
+    elsif @front == @back
+      interim = @queue[@front]
+      @front = -1
+      @back = -1
+
+      return interim 
+    else
+      interim = @queue[@front]
+      @front = (1 + @front) % @size
+
+      return interim
+    end 
   end
 
   def front
-    raise NotImplementedError, "Not yet implemented"
+    return @queue[@front]
+  end
+  
+  @private
+  def get_queue_contents()
+    if @queue.empty?
+      return []
+    elsif @front > @back
+      return @queue[@front...@size] + @queue[0...@back]
+    else
+      return @queue[@front...@back]
+    end
   end
 
   def size
-    raise NotImplementedError, "Not yet implemented"
+    self.get_queue_contents.length
   end
 
   def empty?
-    raise NotImplementedError, "Not yet implemented"
+    return self.get_queue_contents.empty?
   end
 
   def to_s
-    return @store.to_s
+    return self.get_queue_contents.to_s
   end
+
 end
diff --git a/lib/stack.rb b/lib/stack.rb
index cfc6ef0f..1cadf15b 100644
--- a/lib/stack.rb
+++ b/lib/stack.rb
@@ -1,22 +1,25 @@
 class Stack
   def initialize
-    # @store = ...
-    raise NotImplementedError, "Not yet implemented"
+    @stack = LinkedList.new
   end
 
   def push(element)
-    raise NotImplementedError, "Not yet implemented"
+    @stack.add_front(element)
   end
 
   def pop
-    raise NotImplementedError, "Not yet implemented"
+    return nil if self.empty?
+
+    element = stack.remove_front
+
+    return element
   end
 
   def empty?
-    raise NotImplementedError, "Not yet implemented"
+    return @stack.empty?
   end
 
   def to_s
-    return @store.to_s
+    return @stack.to_s
   end
 end