adding stack leetcode problem

lectures/21_hash_tables_II/README.md

@@ -167,3 +167,5 @@ functionality.-->
 - [Leetcode problem 232: Implement Queue using Stacks](https://leetcode.com/problems/implement-queue-using-stacks/). Solution: [p232_queue_using_stacks.cpp](../../leetcode/p232_queue_using_stacks.cpp).
 - [Leetcode problem 102: Binary Tree Level Order Traversal](https://leetcode.com/problems/binary-tree-level-order-traversal/). Solution: [p102_level_order_traversal.cpp](../../leetcode/p102_level_order_traversal.cpp).
   - Play this [animation](https://jidongxiao.github.io/CSCI1200-DataStructures/animations/trees/level_order/index.html) to see how the level order traversal works.
+
+- [Leetcode problem 20: Valid Parentheses](https://leetcode.com/problems/valid-parentheses/). Solution: [p20_valid_parentheses.cpp](../../leetcode/p20_valid_parentheses.cpp)

lectures/22_priority_queues/README.md

@@ -8,8 +8,6 @@
 - What’s a Priority Queue?
 - A Priority Queue as a Heap
 - A Heap as a Vector
-- Building a Heap
-- Heap Sort
 
 ## 22.1 Range Based for Loop
 
@@ -117,9 +115,13 @@ Lambda is new to the C++ language (part of C++11). But lambda is a core piece of
 
 - Draw two different binary heaps with these values: 52 13 48 7 32 40 18 25 4
 
-- Draw several other trees with these values that not binary heaps.
+- Draw several other trees with these values which are not binary heaps.
 
-## 22.7 Leetcode Exercises
+## 22.7 Implementing a Heap with a Vector (instead of Nodes & Pointers)
+
+- The standard library (STL) priority_queue is implemented as a binary heap.
+
+## 22.8 Leetcode Exercises
 
 - [Leetcode problem 215: Kth Largest Element in an Array](https://leetcode.com/problems/rearrange-words-in-a-sentence/). Solution: [p1451_rearrange_words_in_a_sentence.cpp](../../leetcode/p1451_rearrange_words_in_a_sentence.cpp).
 - [Leetcode problem 373: Find K Pairs with Smallest Sums](https://leetcode.com/problems/most-frequent-subtree-sum/). Solution: [p508_most_frequent_subtree_sum.cpp](../../leetcode/p508_most_frequent_subtree_sum.cpp).