"delete_fixup" optimization (keon#873)

* "delete_fixup" optimization

* node_min.parent != node -> node_min.parent is not node

algorithms/tree/red_black_tree/red_black_tree.py

@@ -194,7 +194,7 @@ def delete(self, node):
             node_color = node_min.color
             temp_node = node_min.right
             ## 
-            if node_min.parent != node:
+            if node_min.parent is not node:
                 self.transplant(node_min, node_min.right)
                 node_min.right = node.right
                 node_min.right.parent = node_min
@@ -208,9 +208,9 @@ def delete(self, node):
 
     def delete_fixup(self, node):
         # 4 cases
-        while node != self.root and node.color == 0:
+        while node is not self.root and node.color == 0:
             # node is not root and color is black
-            if node == node.parent.left:
+            if node is node.parent.left:
                 # node is left node
                 node_brother = node.parent.right