diff --git a/lib/problems.rb b/lib/problems.rb
index 5085953d..c0d57951 100644
--- a/lib/problems.rb
+++ b/lib/problems.rb
@@ -1,13 +1,40 @@
-require_relative './stack.rb'
+require_relative './Stack.rb'
 
-# Time Complexity: ?
-# Space Complexity: ?
+# Time Complexity: O(nlogn)
+# Space Complexity: O(n)
 def balanced(string)
-  raise NotImplementedError, "Not implemented yet"
+  stack = Stack.new
+  arr = string.split('')
+  chars = ['(', '[', '{']
+  arr.each do |char|
+    if chars.include?(char)
+      stack.push(char)
+    else
+      current = stack.pop
+      if current == '(' && char != ')'
+        return false
+      elsif current == '[' && char != ']'
+        return false
+      elsif current == '{' && char != '}'
+        return false
+      end
+    end
+  end
+    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 char.match(/[0-9]/)
+      stack.push(char.to_i)
+    else
+      a = stack.pop
+      b = stack.pop
+      stack.push(eval "#{b}#{char}#{a}")
+    end
+  end
+  return stack.pop
 end
diff --git a/lib/queue.rb b/lib/queue.rb
index 828217c6..0c2235ca 100644
--- a/lib/queue.rb
+++ b/lib/queue.rb
@@ -1,31 +1,65 @@
 class Queue
 
-  def initialize
-    # @store = ...
-    raise NotImplementedError, "Not yet implemented"
+  attr_accessor :start, :size, :start, :end
+
+  def initialize(size=20)
+    @store = Array.new(size)
+    @size = size
+    @start, @end = -1, -1
+  end
+
+  def full?
+    (@end + 1) % @size == @start
   end
 
   def enqueue(element)
-    raise NotImplementedError, "Not yet implemented"
+    # raise ArgumentError.new("Queue full") if self.full?
+
+    if self.empty?
+      @start = 0
+      @end = 1
+      @store[@start] = element
+    elsif @start == @end
+      raise ArgumentError.new("Queue full!")
+    else
+      @store[@end] = element
+      @end = (@end + 1) % @size
+    end
   end
 
   def dequeue
-    raise NotImplementedError, "Not yet implemented"
+    if @start == -1
+      raise ArgumentError.new("Queue empty")
+    else
+      element = @store[@start]
+      @start = (@start + 1) % @size
+      if @start == @end
+        @start, @end = -1 
+      end
+    end
+    return element
   end
 
   def front
-    raise NotImplementedError, "Not yet implemented"
+    raise ArgumentError.new("Queue empty") if @start == -1
+    @start
   end
 
   def size
-    raise NotImplementedError, "Not yet implemented"
+    @size
   end
 
   def empty?
-    raise NotImplementedError, "Not yet implemented"
+    return @start == -1
   end
 
   def to_s
-    return @store.to_s
+    arr = []
+    current = @start
+    while current != @end
+      arr << @store[current]
+      current = (current+1) % size
+    end
+    return arr.to_s
   end
 end
diff --git a/lib/stack.rb b/lib/stack.rb
index cfc6ef0f..29a90e42 100644
--- a/lib/stack.rb
+++ b/lib/stack.rb
@@ -1,22 +1,24 @@
+require_relative 'linked_list'
+
 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 if !@store.empty?
   end
 
   def empty?
-    raise NotImplementedError, "Not yet implemented"
+    return @store.empty?
   end
 
   def to_s
     return @store.to_s
   end
 end
+
diff --git a/test/problems_test.rb b/test/problems_test.rb
index 046f059e..2defe612 100644
--- a/test/problems_test.rb
+++ b/test/problems_test.rb
@@ -1,8 +1,9 @@
 require_relative 'test_helper'
+require_relative '../lib/problems.rb' 
 
 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