gcd lcm https://medium.com/%40iamcrypticcoder/a-few-problems-to-remember-of-gcd-and-lcm-a4882cd81094
DFS tree https://codeforces.com/blog/entry/68138#comment-650743
[Tutorial] Non-trivial DP Tricks and Techniques https://codeforces.com/blog/entry/47764
Chicken McNugget Theorem explained[Shortly] https://codeforces.com/blog/entry/118246
[Tutorial] Math note — Möbius inversion https://codeforces.com/blog/entry/53925
General Tricks https://codeforces.com/blog/entry/48417
union of intervals with segment tree: https://codeforces.com/blog/entry/137300
Geometry: 2D points and lines [Tutorial] https://codeforces.com/blog/entry/48122