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 413 / 419 1200
Subtree update 395 / 402 1400
Ancestor 351 / 354 1400
Shallow 318 / 324 1500
Remove subtree 202 / 251 1600
Queries on tree 153 / 242 1900
Queries on tree 2 104 / 126 1900
Library 60 / 75 1900
Tree rotation 34 / 39 1900
Black vertices 62 / 67 2000