Added a double linked list data structure

data_structures/LinkedLists/DoubleList.rb

@@ -0,0 +1,80 @@
+# Define a node in the list
+class Node
+  attr_accessor :value, :next, :prev
+  def initialize(value)
+    @value = value
+    @next = nil
+    @prev = nil
+  end
+end
+
+# A Class for double linked lists (each element links to the next one, and to the previous one)
+class DoubleList
+  attr_accessor :head, :tail
+  def initialize()
+    @head = nil
+    @tail = nil
+  end
+
+  def insert_tail(value)
+    new_node = Node.new(value)
+    if (@head == nil)
+      @head = new_node
+      @tail = new_node
+    else
+      @tail.next = new_node
+      new_node.prev = @tail
+      @tail = new_node
+    end
+  end
+
+  def insert_head(value)
+    new_node = Node.new(value)
+    if (@head == nil)
+      @head = new_node
+      @tail = new_node
+    else
+      new_node.next = @head
+      @head.prev = new_node
+      @head = new_node
+    end
+  end
+
+  def delete_tail()
+    if (@tail != nil)
+      @tail = @tail.prev
+      if (@tail != nil)
+        @tail.next = nil
+      end
+    end
+  end
+
+  def delete_head()
+    if (@head != nil)
+      @head = @head.next
+      if (@head != nil)
+        @head.prev = nil
+      end
+    end
+  end
+
+  def print_list()
+    print "["
+    if (@head != nil)
+      printNode = @head
+      while (printNode != nil)
+        print "#{printNode.value}"
+        if (printNode != @tail)
+          print ", "
+        end
+        printNode = printNode.next
+      end
+    end
+    print "]"
+    STDOUT.flush
+  end
+
+  def is_empty()
+    return (@head==nil)
+  end
+end