add popHead and Extend functions to linked list data structure

lib/common/linkedlist.js

@@ -19,6 +19,36 @@ module.exports = function () {
     }
     return node;
   };
+
+  list.popHead = function () {
+    var result = list.head;
+    if (list.head != null) {
+      list.head = list.head.next;
+      if (list.head == null) {
+        list.tail = null;
+      } else {
+        list.head.previous  = null;
+      }
+    }
+    return result;
+  };
+
+  // merges two linked lists and return a pointer to the head of the merged list
+  // the head will be the head of list and the tail the tail of l2
+  list.extend = function (l2) {
+    if (list.head == null) {
+      return l2;
+    }
+    if (l2.head == null) {
+      return list;
+    }
+    list.tail.next = l2.head;
+    l2.head.previous = list.tail;
+    list.tail = l2.tail;
+
+    return list;
+  };
+
   list.remove = function (ptr) {
     var prev = ptr.previous;
     var next = ptr.next;
@@ -83,4 +113,4 @@ module.exports = function () {
   };
   */
   return list;
-};
+};