Added another stack video.

plan.txt

@@ -212,6 +212,7 @@ Then test it out on a computer to make sure it's not buggy from syntax.
 Stacks
     - see: https://class.coursera.org/algs4partI-010/lecture
     - https://www.coursera.org/learn/data-structures/lecture/UdKzQ/stacks
+    - https://class.coursera.org/algs4partI-010/lecture/18
 Queues
     - see: https://class.coursera.org/algs4partI-010/lecture
     - https://www.coursera.org/learn/data-structures/lecture/EShpq/queues