A collection of C++ libraries I've been personally maintaining and using in solving competitive programming problems. Some I frequently used or having high quality are listed as below:

Math/ModInt.h: A ModInt type to deal with intergers having to mod some number(e.g 1e9+7)

Graph/max flow.h: A Dinic's implementation for computing max flow

Search/GetID.h: A library to assign unique id to strings or objects of other types

Search/UnionFind.h: A implemenation of union find disjoint set

Search/Segment tree(Iterative).h: A general purpose segment tree template with lazy propagation(support non-commutative updates)

Search/square root decomposition.h: A general purpose square root decomposition data structure template

Search/Treap.h: A Treap implementation that is configurable with toggles on lazy propagation, supporting size and sum.