FFT
Li-Chao Segment Tree
Segment Tree Beats
RMQ in O(n)/O(1)
Any self-balancing tree except treap
Link-cut tree
Wavelet tree
Mergesort tree
Binomial heap
Fibonacci heap
Leftist heap
Dominator tree
3-connected components in O(n)
k-th shortest path
Matching in general graph
Weighted matching in general graph
Preflow-push
MCMF in O(poly(V,E))
Minimum arborescence (directed MST) in O(ElogV)
Suffix tree
Online convex hull in 2D
Convex hull in 3D
Halfplane intersection
Voronoi diagram / Delaunay triangulation
Operation on formal power series (exp, log, sqrt, ...)
How to actually use generating functions to solve problems
Lagrange Inversion formula
Sweepline Mo
Matroid intersection
What's the easiest concept you ever learned in tech ?