Updated isEmpty function definition. (#31)

'return len(self.stack) == []' is a problematic definition. it should be either  'return len(self.stack) == 0' or 'self.stack == []'

Stack/Stack.ipynb

@@ -107,7 +107,7 @@
     "        \n",
     "    # to check if stack is empty\n",
     "    def isEmpty(self):\n",
-    "        return len(self.stack) == []\n",
+    "        return len(self.stack) == 0\n",
     "    \n",
     "    # for checking the size of stack\n",
     "    def size(self):\n",