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 379 / 385 1200
Subtree update 360 / 368 1400
Ancestor 322 / 325 1400
Shallow 294 / 299 1500
Remove subtree 186 / 232 1600
Queries on tree 135 / 220 1900
Queries on tree 2 93 / 115 1900
Library 57 / 73 1900
Tree rotation 26 / 30 1900
Black vertices 56 / 61 2000