Added FFT and a couple more patterns.

plan.txt

@@ -159,6 +159,8 @@ design patterns:
     - adapter
     - prototype
     - decorator
+    - visitor
+    - factory
 Combinatorics (n choose k)
 Probability
 Dynamic Programming
@@ -205,6 +207,9 @@ Bloom Filter
     - https://www.youtube.com/watch?v=-SuTGoFYjZs
     - http://blog.michaelschmatz.com/2016/04/11/how-to-write-a-bloom-filter-cpp/
 
+Fast Fourier Transform
+    - http://jakevdp.github.io/blog/2013/08/28/understanding-the-fft/
+
 C (for basis of C)
 C++ (for interview answers)