Module Euler tour

Euler tour

**Frequency: 7/10** Also known as DFS order. This is important for solving tree related problems from now on.

Resources

- [USACO: Euler Tour](https://usaco.guide/gold/tree-euler)

Problems

Subtree queries 411 / 417 1200
Subtree update 393 / 400 1400
Ancestor 349 / 352 1400
Shallow 316 / 322 1500
Remove subtree 202 / 251 1600
Queries on tree 152 / 239 1900
Queries on tree 2 104 / 126 1900
Library 60 / 75 1900
Tree rotation 34 / 39 1900
Black vertices 62 / 67 2000