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 293 / 298 1200
Subtree update 285 / 292 1400
Ancestor 253 / 255 1400
Shallow 230 / 234 1500
Remove subtree 147 / 191 1600
Queries on tree 106 / 166 1900
Queries on tree 2 75 / 88 1900
Library 50 / 62 1900
Tree rotation 9 / 10 1900
Black vertices 47 / 52 2000