moving image to lecture 20

lectures/19_trees_II/README.md

@@ -86,7 +86,7 @@ point to the “in-order predecessor”.
   – Each node stores a parent pointer. Only the root node has a null parent pointer. [method 1]
 
 
-  – Each iterator maintains a stack of pointers representing the path down the tree to the current node. [method 2]
+  – Each iterator maintains a container of pointers representing the path down the tree to the current node. [method 2]
 
 
 - If we choose the parent pointer method, we’ll need to rewrite the insert and erase member functions to
@@ -99,11 +99,12 @@ Be sure to draw a picture to help you understand!
 ![alt text](ds_set_parent_ptrs.png "ds set parent ptrs")
 
 Exercise: [method 2] Write a fragment of code that given a tree iterator containing a pointer to the node and a
-stack of pointers representing path from root to node, finds the in-order successor (without using parent pointers).
-Either version can be extended to complete the implementation of increment/decrement for the ds_set tree iterators.
-![alt text](ds_set_history.png "ds set history")
+container of pointers representing path from root to node, finds the in-order successor (without using parent pointers).
+Either version can be extended to complete the implementation of increment/decrement for the ds_set tree iterators. (This method will be covered in the next lecture)
 
-Exercise: What are the advantages & disadvantages of each method?
+<!-- ![alt text](ds_set_history.png "ds set history") -->
+
+<!-- Exercise: What are the advantages & disadvantages of each method? -->
 
 ## 19.5 Limitations of Our BST Implementation