Update 145-binary_tree_postorder_traversal.py

Author: stevestar888

Diff for: 145-binary_tree_postorder_traversal.py

@@ -1,4 +1,9 @@
 """
+https://leetcode.com/problems/binary-tree-postorder-traversal/
+
+Strat: 
+    Do recursively. Iterative needs additional space or destroying the tree.
+    
 Post : Left Right Me
 
     4
@@ -49,4 +54,4 @@ def postorder_helper(self, root, result):
 #         curr = root
 
 #         while stack or curr:
-#             pass
+#             pass