Update README - companies retrieved on December 2020

Author: Sean Prashad

README.md

@@ -56,7 +56,7 @@ The entire question list can be found here:
 https://seanprashad.com/leetcode-patterns/.
 
 In addition to viewing the question list, companies that have previously asked
-the question in the past 6 months (_as of January 2020_) will be listed. You can
+the question in the past 6 months (_as of December 2020_) will be listed. You can
 also use the checkboxes to mark which questions you've completed!
 
 ## Solutions
@@ -92,6 +92,10 @@ If given a linked list then
 If recursion is banned then
     - Stack
 
+If must solve in-place then
+    - Swap corresponding values
+    - Store one or more different values in the same pointer
+
 If asked for maximum/minumum subarray/subset/options then
     - Dynamic programming