Add algorithm playground (#10923)

* Add algorithm playgroud

* fix alphabetical order

* Update more/free-programming-playgrounds.md

* Update more/free-programming-playgrounds.md

---------

Co-authored-by: Eric Hellman <eric@hellman.net>

more/free-programming-playgrounds.md

@@ -1,5 +1,6 @@
 ### Index
 
+* [Algorithms](#algorithms)
 * [APL](#apl)
 * [Assembly](#assembly)
 * [Bash](#bash)
@@ -53,6 +54,12 @@
 * [Vim](#vim)
 
 
+### Algorithms
+
+* [Data Structures Visualization](https://www.cs.usfca.edu/~galles/visualization/Algorithms.html) - David Galles
+* [The Algorithm Visualizer](https://algorithm-visualizer.org)
+
+
 ### APL
 
 * [APLgolf](https://razetime.github.io/APLgolf)