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 287 / 292 1200
Subtree update 280 / 286 1400
Ancestor 250 / 252 1400
Shallow 228 / 232 1500
Remove subtree 146 / 190 1600
Queries on tree 103 / 164 1900
Queries on tree 2 73 / 84 1900
Library 49 / 61 1900
Tree rotation 8 / 10 1900
Black vertices 47 / 52 2000