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 410 / 416 1200
Subtree update 391 / 399 1400
Ancestor 347 / 350 1400
Shallow 315 / 321 1500
Remove subtree 199 / 247 1600
Queries on tree 148 / 234 1900
Queries on tree 2 102 / 124 1900
Library 60 / 75 1900
Tree rotation 32 / 36 1900
Black vertices 61 / 65 2000