diff --git a/lib/problems.rb b/lib/problems.rb
index 5085953d..1c3eb9fc 100644
--- a/lib/problems.rb
+++ b/lib/problems.rb
@@ -1,13 +1,52 @@
 require_relative './stack.rb'
 
-# Time Complexity: ?
-# Space Complexity: ?
+# Time Complexity: O(n) - n is chars in string
+# Space Complexity: O(1/2n) --> O(n) - n is chars in string
 def balanced(string)
-  raise NotImplementedError, "Not implemented yet"
+  return true if string.empty?
+  return false if string.length % 2 != 0
+  
+  s = Stack.new
+  open_bracs = ["(", "[", "{"]
+  close_bracs = [")", "]", "}"]
+  
+  string.each_char do |char|
+    if open_bracs.include?(char)
+      s.push(char)
+    elsif close_bracs.include?(char)
+      compare = s.pop
+      if open_bracs.find_index(compare) != close_bracs.find_index(char)
+        return false
+      end
+    end
+  end
+  
+  return true
 end
 
 # Time Complexity: ?
 # Space Complexity: ?
 def evaluate_postfix(postfix_expression)
-  raise NotImplementedError, "Not implemented yet"
+  operands = ["*", "**", "/", "%", "+", "-"]
+  s= Stack.new
+  
+  postfix_expression.each_char do |char|
+    if operands.include?(char)
+      first = s.pop
+      second = s.pop
+      result = eval("#{second}#{char}#{first}")
+      s.push(result)
+    else
+      s.push(char)
+    end
+  end
+  
+  return s.pop
 end
+
+# p balanced("{[{}{}]}")
+# p balanced("{[{}{}]")
+# p balanced("{()}[)")
+
+# p evaluate_postfix("35+6*") #48
+# p evaluate_postfix("53+62/*35*+") #39
\ No newline at end of file
diff --git a/lib/queue.rb b/lib/queue.rb
index 828217c6..14f81ce4 100644
--- a/lib/queue.rb
+++ b/lib/queue.rb
@@ -1,31 +1,72 @@
 class Queue
-
+  
   def initialize
-    # @store = ...
-    raise NotImplementedError, "Not yet implemented"
+    @store = Array.new(50)
+    @front = -1
+    @back = -1
   end
-
+  
   def enqueue(element)
-    raise NotImplementedError, "Not yet implemented"
+    if((@front == 0 && @back == @store.length - 1) || (@front == @back + 1))
+      raise ArgumentError
+    elsif @front == -1 && @back == -1
+      @front = 0
+      @back = 0
+    end
+    
+    @store[@back] = element
+    @back = (@back + 1) % @store.length
   end
-
+  
   def dequeue
-    raise NotImplementedError, "Not yet implemented"
+    if @front == -1
+      raise ArgumentError
+    end
+    
+    data = @store[@front]
+    @store[@front] = nil
+    @front = (@front + 1) % @store.length
+    
+    if @front == @back
+      @front = -1
+      @back = -1
+    end
+    
+    return data
   end
-
+  
   def front
-    raise NotImplementedError, "Not yet implemented"
+    return @store[@front]
   end
-
+  
   def size
-    raise NotImplementedError, "Not yet implemented"
+    if self.empty?
+      raise ArgumentError
+    end
+    
+    return @back - @front
   end
-
+  
   def empty?
-    raise NotImplementedError, "Not yet implemented"
+    if @front == -1 && @back == -1
+      return true
+    else
+      return false
+    end
   end
-
+  
   def to_s
-    return @store.to_s
+    return @store[@front...@back].to_s
   end
 end
+
+# q = Queue.new
+# q.enqueue(0)
+# q.enqueue(1)
+# q.enqueue(2)
+# q.enqueue(3)
+# p q
+# p q.size
+# p q.dequeue
+# p q.size
+# p q.front
\ No newline at end of file
diff --git a/lib/stack.rb b/lib/stack.rb
index cfc6ef0f..d1ca0c0c 100644
--- a/lib/stack.rb
+++ b/lib/stack.rb
@@ -1,22 +1,30 @@
+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"
+    return nil if self.empty?
+    element = @store.remove_last()
+    return element
   end
-
+  
   def empty?
-    raise NotImplementedError, "Not yet implemented"
+    return @store.empty?
   end
-
+  
   def to_s
     return @store.to_s
   end
 end
+
+# stack = Stack.new
+# stack.push(5)
+# stack.push(10)
+# p stack
\ No newline at end of file