Starter Pack

A lot of standard optimization tricks and some advanced stuff : https://robert1003.github.io/

Basic Graphs:
  Shortest Paths : https://codeforces.com/blog/entry/45897

DP:
  Basic DP : https://codeforces.com/blog/entry/43256, https://codeforces.com/blog/entry/67679
  Tree DP : https://codeforces.com/blog/entry/20935
  Digit DP : https://codeforces.com/blog/entry/77096
  Advanced Tricks : https://codeforces.com/blog/entry/47764
  Alien's Trick : https://codeforces.com/blog/entry/68778, https://codeforces.com/blog/entry/68778#comment-700891

Trees:
  Path updates using fenwick trees : https://codeforces.com/blog/entry/78564
  Techniques for tree problems : https://codeforces.com/blog/entry/88669
  Reachability Trees : https://codeforces.com/blog/entry/85714
  Centroid Decomposition : 
    https://medium.com/carpanese/an-illustrated-introduction-to-centroid-decomposition-8c1989d53308
    https://codeforces.com/blog/entry/52492
    https://threadsiiithyderabad.quora.com/Centroid-Decomposition-of-a-Tree
  Heavy Light Decomposition : 
    https://codeforces.com/blog/entry/81317
    https://blog.anudeep2011.com/heavy-light-decomposition/
    
Math:
  Basics NT : https://codeforces.com/blog/entry/46620
  Catalan Numbers : https://codeforces.com/blog/entry/87585
  PIE : https://codeforces.com/blog/entry/64625
  
Game Theory:
  https://codeforces.com/blog/entry/66040
  https://codeforces.com/blog/entry/85984

Bitwise fiddling hacks : https://graphics.stanford.edu/~seander/bithacks.html